ipw2x00: 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/220
Link: https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
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/YzIeULWc17XSIglv@work

drivers/net/wireless/intel/ipw2x00/ipw2200.h

@@ -651,7 +651,7 @@ struct ipw_rx_notification {
 		struct notif_link_deterioration link_deterioration;
 		struct notif_calibration calibration;
 		struct notif_noise noise;
-		u8 raw[0];
+		DECLARE_FLEX_ARRAY(u8, raw);
 	} u;
 } __packed;