Fix the following kernel warning that occurs if scan work is scheduled
and the hw scan has been cancelled by mac80211

WARNING: CPU: 1 PID: 502 at net/mac80211/scan.c:391 __ieee80211_scan_completed+0x299/0x690
Modules linked in:
CPU: 1 PID: 502 Comm: kworker/u4:4 Not tainted 5.7.0-rc1+ #2882
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.13.0-2.fc32 04/01/2014
Workqueue: phy0 ieee80211_scan_work
RIP: 0010:__ieee80211_scan_completed+0x299/0x690
RSP: 0018:ffffc9000036fda8 EFLAGS: 00010246
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 8c6318c6318c6320
RDX: 0000000000000000 RSI: 00000000ffffffff RDI: ffff88803bb129b8
RBP: ffff88803bb10d80 R08: 0000000000000001 R09: 0000000000000001
R10: 0000000000000001 R11: 0000000000000000 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: ffff88803bb12ae8
FS:  0000000000000000(0000) GS:ffff88803ec00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f13ee5c1470 CR3: 000000003c790000 CR4: 00000000000006a0
Call Trace:
 ieee80211_scan_work+0x170/0x850
 ? sched_clock_cpu+0x11/0xb0
 process_one_work+0x24f/0x580
 ? worker_thread+0xcc/0x3e0
 worker_thread+0x4f/0x3e0
 ? process_one_work+0x580/0x580
 kthread+0x11b/0x140
 ? __kthread_bind_mask+0x60/0x60
 ret_from_fork+0x3a/0x50
irq event stamp: 9255532
hardirqs last  enabled at (9255531): [<ffffffff81a504e4>] _raw_spin_unlock_irq+0x24/0x30
hardirqs last disabled at (9255532): [<ffffffff81001b3f>] trace_hardirqs_off_thunk+0x1a/0x1c
softirqs last  enabled at (9255526): [<ffffffff8107c9ef>] process_one_work+0x24f/0x580
softirqs last disabled at (9255524): [<ffffffff815f2f41>] mt7615_scan_work+0x91/0xc0
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>