wifi: mwifiex: Replace one-element array with flexible-array member in struct mwifiex_ie_types_rxba_sync

One-element and zero-length arrays are deprecated. So, replace
one-element array in struct mwifiex_ie_types_rxba_sync with
flexible-array member, and refactor the rest of the code, accordingly.

This results in no differences in binary output.
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/79c801c69c8beece2f80502c60166036d3c047cc.1692931954.git.gustavoars@kernel.org