wifi: carl9170: Replace zero-length array of trailing structs with flex-array

Zero-length arrays are deprecated[1] and are being replaced with
flexible array members in support of the ongoing efforts to tighten the
FORTIFY_SOURCE routines on memcpy(), correctly instrument array indexing
with UBSAN_BOUNDS, and to globally enable -fstrict-flex-arrays=3.

Replace zero-length array with flexible-array member.

This results in no differences in binary output.

[1] KSPP#78

Cc: Christian Lamparter <[email protected]>
Cc: Kalle Valo <[email protected]>
Cc: "David S. Miller" <[email protected]>
Cc: Eric Dumazet <[email protected]>
Cc: Jakub Kicinski <[email protected]>
Cc: Paolo Abeni <[email protected]>
Cc: "Gustavo A. R. Silva" <[email protected]>
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Kees Cook <[email protected]>
Reviewed-by: Gustavo A. R. Silva <[email protected]>
Acked-by: Christian Lamparter <[email protected]>
Signed-off-by: Kalle Valo <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: kees

drivers/net/wireless/ath/carl9170/fwcmd.h

@@ -118,10 +118,10 @@ struct carl9170_reg_list {
 } __packed;
 
 struct carl9170_write_reg {
-	struct {
+	DECLARE_FLEX_ARRAY(struct {
 		__le32		addr;
 		__le32		val;
-	} regs[0] __packed;
+	} __packed, regs);
 } __packed;
 
 struct carl9170_write_reg_byte {