ath11k: convert ath11k_wmi_pdev_set_ps_mode() to use enum wmi_sta_ps_mode

It's more descriptive to use the actual enum used by the firmware instead of a
boolean so change ath11k_wmi_pdev_set_ps_mode() to use a boolean.

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/20211108123826.8463-1-kvalo@codeaurora.org