wifi: rtw89: pci: add .shutdown callback to stop rfkill polling on reboot

In the Linux kernel, the following vulnerability has been resolved:

wifi: rtw89: pci: add .shutdown callback to stop rfkill polling on reboot

Since the hardware rfkill polling was introduced, arm64 platforms can
panic with an asynchronous SError during warm reboot:

SError Interrupt on CPU8, code 0x00000000be000011 -- SError
Workqueue: events_power_efficient rfkill_poll [rfkill]
rtw89_pci_ops_read8+0x94/0x160 [rtw89_pci]
rtw89_core_rfkill_poll+0x50/0x1e0 [rtw89_core]
rtw89_ops_rfkill_poll+0x40/0x68 [rtw89_core]
ieee80211_rfkill_poll+0x3c/0x70 [mac80211]
cfg80211_rfkill_poll+0x40/0x2a0 [cfg80211]
rfkill_poll+0x30/0x88 [rfkill]
Kernel panic - not syncing: Asynchronous SError Interrupt

On the reboot path the kernel only runs device_shutdown(), which calls
each driver's .shutdown callback; .remove is not invoked. The rtw89 PCI
driver had no .shutdown callback, so nothing stopped the rfkill polling
work while the platform was tearing the PCIe link down. Once the link
is gone, the next MMIO read from the poll handler targets a
non-responding device and is reported as a fatal asynchronous SError on
arm64.

Add rtw89_pci_shutdown(), wired to all rtw89 PCI device drivers, which
sets a new RTW89_FLAG_SHUTDOWN flag (mirroring the USB
RTW89_FLAG_UNPLUGGED pattern). When the flag is set,
rtw89_ops_rfkill_poll() returns early, so no MMIO read is issued to the
chip after shutdown begins and the SError no longer occurs.

This does not call the full .remove path from .shutdown, to keep the
shutdown handler minimal and avoid running the non-idempotent teardown
twice.