ath10k: replenish htc tx credits always

There's always at most 2 credits and it makes
little sense to set the
ATH10K_HTC_FLAG_NEED_CREDIT_UPDATE flag
conditionally.

This seems to fix some random issues with tx
credit starvation on WLAN.RM.2.0-00073 I've been
seeing. Note: this isn't related to wmi mgmt tx.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>