wifi: ath10k: Use DECLARE_FLEX_ARRAY() for ath10k_htc_record

Transform the zero-length arrays in ath10k_htc_record into proper
flexible arrays via the DECLARE_FLEX_ARRAY() macro. This helps with
ongoing efforts to globally enable -Warray-bounds.

Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20231127-flexarray-htc_record-v1-2-6be1f36126fd@quicinc.com