For QCA6390, Start a timer to update CE pipe 4 ring HP when shadow
register is enabled. Its' to avoid that HP isn't updated to target
register.

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/1601544890-13450-7-git-send-email-kvalo@codeaurora.org