wifi: ath12k: Fix uninitilized variable clang warnings

Fix uninitilized variable warnings spotted during clang compilation.

Warnings:
drivers/net/wireless/ath/ath12k/mac.c:1076:8: warning: variable 'ret' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
drivers/net/wireless/ath/ath12k/dbring.c:272:7: warning: variable 'ring' is used uninitialized whenever switch case is taken [-Wsometimes-uninitialized]
drivers/net/wireless/ath/ath12k/dp_rx.c:2690:34: warning: variable 'i' is uninitialized when used here [-Wuninitialized]

Tested-on: QCN9274 hw2.0 PCI WLAN.WBE.1.0-03171-QCAHKSWPL_SILICONZ-1
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Thiraviyam Mariyappan <quic_tmariyap@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20221221105529.19345-2-quic_tmariyap@quicinc.com