Fix a corner case between PCI device driver remove callback and
runtime PM idle callback.

Following sequence of events can happen:
  - at azx_create, context is allocated with devm_kzalloc() and
    stored as pci_set_drvdata()
  - user-space requests to unbind audio driver
  - dd.c:__device_release_driver() calls PCI remove
  - pci-driver.c:pci_device_remove() calls the audio
    driver azx_remove() callback and this is completed
  - pci-driver.c:pm_runtime_put_sync() leads to a call
    to rpm_idle() which again calls azx_runtime_idle()
  - the azx context object, as returned by dev_get_drvdata(),
    is no longer valid
  -> access fault in azx_runtime_idle when executing
	struct snd_card *card = dev_get_drvdata(dev);
	chip = card->private_data;
	if (chip->disabled || hda->init_failed)

This was discovered by i915_module_load test with 5.15.0 based
linux-next tree.

Example log caught by i915_module_load test with linux-next
https://intel-gfx-ci.01.org/tree/linux-next/

<4> [264.038232] general protection fault, probably for non-canonical address 0x6b6b6b6b6b6b73f0: 0000 [#1] PREEMPT SMP NOPTI
<4> [264.038248] CPU: 0 PID: 5374 Comm: i915_module_loa Not tainted 5.15.0-next-20211109-gc8109c2ba35e-next-20211109 #1
[...]
<4> [264.038267] RIP: 0010:azx_runtime_idle+0x12/0x60 [snd_hda_intel]
[...]
<4> [264.038355] Call Trace:
<4> [264.038359]  <TASK>
<4> [264.038362]  __rpm_callback+0x3d/0x110
<4> [264.038371]  rpm_idle+0x27f/0x380
<4> [264.038376]  __pm_runtime_idle+0x3b/0x100
<4> [264.038382]  pci_device_remove+0x6d/0xa0
<4> [264.038388]  device_release_driver_internal+0xef/0x1e0
<4> [264.038395]  unbind_store+0xeb/0x120
<4> [264.038400]  kernfs_fop_write_iter+0x11a/0x1c0

Fix the issue by setting drvdata to NULL at end of azx_remove().
Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Link: https://lore.kernel.org/r/20211110210307.1172004-1-kai.vehmanen@linux.intel.comSigned-off-by: Takashi Iwai <tiwai@suse.de>