wifi: wil6210: wmi: Replace zero-length array with DECLARE_FLEX_ARRAY() helper

Zero-length arrays are deprecated, and we are moving towards adopting
C99 flexible-array members, instead. So, replace zero-length arrays
declarations alone in structs with the new DECLARE_FLEX_ARRAY()
helper macro.

This helper allows for flexible-array members alone in structs.

Link: https://github.com/KSPP/linux/issues/193
Link: https://github.com/KSPP/linux/issues/288
Link: https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/ZGKHM+MWFsuqzTjm@work