wifi: ath11k: Fix ath11k_htc_record flexible record

Transform the zero-length ath11k_htc_record::credit_report array into
a proper flexible array. Since this is the only array in
ath11k_htc_record, remove the unnecessary union.

Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20231205-flexarray-htc_record-v2-1-fbb56d436951@quicinc.com