ath11k: mhi: print a warning if firmware crashed

There was no way to detect if the firmware crashed so add a warning. At the
moment the firmware is not restarted or anything like that, so when this
happens ath11k modules need to be reloaded.

Tested-on: QCA6390 hw2.0 PCI WLAN.HST.1.0.1-01740-QCAHSTSWPLZ_V2_TO_X86-1

Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1607609124-17250-2-git-send-email-kvalo@codeaurora.org