ath11k: set credit_update flag for flow controlled ep only

This change is required because firmware will check all the pipes
before entering WoW. If ATH11K_HTC_FLAG_NEED_CREDIT_UPDATE is set,
firmware treats this pipe needed to return credit even though it's
actually not required. If any pipe needs to return credit, the
suspend_complete message doesn't send to host but is dropped.
So host gets time out and WoW suspend failed.

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>