Fix kernel crash when the firmware is missing or fails to download.

[    9.444758] kernel BUG at drivers/pci/msi.c:375!
[    9.449363] Internal error: Oops - BUG: 0 [#1] PREEMPT SMP
[    9.501033] pstate: a0400009 (NzCv daif +PAN -UAO)
[    9.505814] pc : free_msi_irqs+0x180/0x184
[    9.509897] lr : free_msi_irqs+0x40/0x184
[    9.513893] sp : ffffffc015193870
[    9.517194] x29: ffffffc015193870 x28: 00000000f0e94fa2
[    9.522492] x27: 0000000000000acd x26: 000000000000009a
[    9.527790] x25: ffffffc0152cee58 x24: ffffffdbb383e0d8
[    9.533087] x23: ffffffdbb38628d0 x22: 0000000000040200
[    9.538384] x21: ffffff8cf7de7318 x20: ffffff8cd65a2480
[    9.543681] x19: ffffff8cf7de7000 x18: 0000000000000000
[    9.548979] x17: ffffff8cf9ca03b4 x16: ffffffdc13ad9a34
[    9.554277] x15: 0000000000000000 x14: 0000000000080800
[    9.559575] x13: ffffff8cd65a2980 x12: 0000000000000000
[    9.564873] x11: ffffff8cfa45d820 x10: ffffff8cfa45d6d0
[    9.570171] x9 : 0000000000000040 x8 : ffffff8ccef1b780
[    9.575469] x7 : aaaaaaaaaaaaaaaa x6 : 0000000000000000
[    9.580766] x5 : ffffffdc13824900 x4 : ffffff8ccefe0000
[    9.586063] x3 : 0000000000000000 x2 : 0000000000000000
[    9.591362] x1 : 0000000000000125 x0 : ffffff8ccefe0000
[    9.596660] Call trace:
[    9.599095]  free_msi_irqs+0x180/0x184
[    9.602831]  pci_disable_msi+0x100/0x130
[    9.606740]  pci_free_irq_vectors+0x24/0x30
[    9.610915]  mt7921_pci_probe+0xbc/0x250 [mt7921e]
[    9.615693]  pci_device_probe+0xd4/0x14c
[    9.619604]  really_probe+0x134/0x2ec
[    9.623252]  driver_probe_device+0x64/0xfc
[    9.627335]  device_driver_attach+0x4c/0x6c
[    9.631506]  __driver_attach+0xac/0xc0
[    9.635243]  bus_for_each_dev+0x8c/0xd4
[    9.639066]  driver_attach+0x2c/0x38
[    9.642628]  bus_add_driver+0xfc/0x1d0
[    9.646365]  driver_register+0x64/0xf8
[    9.650101]  __pci_register_driver+0x6c/0x7c
[    9.654360]  init_module+0x28/0xfdc [mt7921e]
[    9.658704]  do_one_initcall+0x13c/0x2d0
[    9.662615]  do_init_module+0x58/0x1e8
[    9.666351]  load_module+0xd80/0xeb4
[    9.669912]  __arm64_sys_finit_module+0xa8/0xe0
[    9.674430]  el0_svc_common+0xa4/0x16c
[    9.678168]  el0_svc_compat_handler+0x2c/0x40
[    9.682511]  el0_svc_compat+0x8/0x10
[    9.686076] Code: a94257f6 f9400bf7 a8c47bfd d65f03c0 (d4210000)
[    9.692155] ---[ end trace 7621f966afbf0a29 ]---
[    9.697385] Kernel panic - not syncing: Fatal exception
[    9.702599] SMP: stopping secondary CPUs
[    9.706549] Kernel Offset: 0x1c03600000 from 0xffffffc010000000
[    9.712456] PHYS_OFFSET: 0xfffffff440000000
[    9.716625] CPU features: 0x080026,2a80aa18
[    9.720795] Memory Limit: none

Fixes: 5c14a5f9 ("mt76: mt7921: introduce mt7921e support")
Reported-by: Claire Chang <tientzu@google.com>
Co-developed-by: YN Chen <YN.Chen@mediatek.com>
Signed-off-by: YN Chen <YN.Chen@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>