ASoC: SOF: control.h: 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 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/211
Link: https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Link: https://lore.kernel.org/r/YzIcZ11k8RiQtS2T@work
Signed-off-by: Mark Brown <[e-mail]>

include/sound/sof/control.h

···
 	/* control data - add new types if needed */
 	union {
 		/* channel values can be used by volume type controls */
-		struct sof_ipc_ctrl_value_chan chanv[0];
+		DECLARE_FLEX_ARRAY(struct sof_ipc_ctrl_value_chan, chanv);
 		/* component values used by routing controls like mux, mixer */
-		struct sof_ipc_ctrl_value_comp compv[0];
+		DECLARE_FLEX_ARRAY(struct sof_ipc_ctrl_value_comp, compv);
 		/* data can be used by binary controls */
-		struct sof_abi_hdr data[0];
+		DECLARE_FLEX_ARRAY(struct sof_abi_hdr, data);
 	};
 } __packed;