ath11k: Silence clang -Wsometimes-uninitialized in ath11k_update_per_peer_stats_from_txcompl

Clang warns a few times (trimmed for brevity):

../drivers/net/wireless/ath/ath11k/debugfs_sta.c:185:7: warning:
variable 'rate_idx' is used uninitialized whenever 'if' condition is
false [-Wsometimes-uninitialized]

It is not wrong, rate_idx is only initialized in the first if block.
However, this is not necessarily an issue in practice because rate_idx
will only be used when initialized because
ath11k_accumulate_per_peer_tx_stats only uses rate_idx when flags is not
set to RATE_INFO_FLAGS_HE_MCS, RATE_INFO_FLAGS_VHT_MCS, or
RATE_INFO_FLAGS_MCS. Still, it is not good to stick uninitialized values
into another function so initialize it to zero to prevent any issues
down the line.

Fixes: d5c65159 ("ath11k: driver for Qualcomm IEEE 802.11ax devices")
Link: https://github.com/ClangBuiltLinux/linux/issues/832


Reported-by:  <ci_notify@linaro.org>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>