ath11k: htc: implement suspend handling

When ath11k sends suspend command to firmware, firmware will
return suspend_complete events and add handlers for those.

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

Signed-off-by: Carl Huang <cjhuang@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1607708150-21066-6-git-send-email-kvalo@codeaurora.org