ASoC: uapi: Replace zero-length arrays 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 in anonymous union with the new __DECLARE_FLEX_ARRAY()
helper macro.

This helper allows for flexible-array members in unions.

Link: https://github.com/KSPP/linux/issues/193
Link: https://github.com/KSPP/linux/issues/227
Link: https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html


Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Amadeusz Sławiński <amadeuszx.slawinski@linux.intel.com>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>