wifi: ath10k: Set tx credit to one for WCN3990 snoc based devices

Currently host can send two WMI commands at once. There is possibility to
cause SMMU issues or corruption, if host wants to initiate 2 DMA
transfers, it is possible when copy complete interrupt for first DMA
reaches host, CE has already updated SRRI (Source ring read index) for
both DMA transfers and is in the middle of 2nd DMA. Host uses SRRI
(Source ring read index) to interpret how many DMA’s have been completed
and tries to unmap/free both the DMA entries. Hence now it is limiting to
one.Because CE is  still in the middle of 2nd DMA which can cause these
issues when handling two DMA transfers.

This change will not impact other targets, as it is only for WCN3990.

Tested-on: WCN3990 hw1.0 SNOC WLAN.HL.2.0-01387-QCAHLSWMTPLZ-1

Signed-off-by: Youghandhar Chintala <quic_youghand@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20220801134941.15216-1-quic_youghand@quicinc.com