mlxfw: 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/285
Link: https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Tested-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/ZGKGiBxP0zHo6XSK@work
Signed-off-by: Jakub Kicinski <kuba@kernel.org>