ath11k: enable idle power save mode

Host sends wmi command to allow hardware enter idle power
save mode in ath11k_mac_op_start function.

hw parameter idle_ps indicates whether idle power save is supported.

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/1601544890-13450-8-git-send-email-kvalo@codeaurora.org