can: etas_es58x: 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 array
declaration in union es58x_urb_cmd with the new DECLARE_FLEX_ARRAY()
helper macro.

This helper allows for a flexible-array member in a union.

Link: https://github.com/KSPP/linux/issues/193
Link: https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.htmlSigned-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Acked-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/all/Yw00w6XRcq7B6ub6@workSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/usb/etas_es58x/es58x_core.h

@@ -222,7 +222,7 @@ union es58x_urb_cmd {
 		u8 cmd_type;
 		u8 cmd_id;
 	} __packed;
-	u8 raw_cmd[0];
+	DECLARE_FLEX_ARRAY(u8, raw_cmd);
 };
 
 /**