ath11k: start a timer to update TCL HP

The timer is to check if TCL HP isn't updated to target.
The timer will postpone itself if there are TX operations
during the interval, otherwise the timer handler updates
the HP again so the index value in HP register will be
forwarded to target register, and the timer stops afterwards.

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/1601544890-13450-5-git-send-email-kvalo@codeaurora.org