adm80211: Replace zero-length array with flexible-array member

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200319225002.GA28673@embeddedor.com

drivers/net/wireless/admtek/adm8211.h

@@ -531,7 +531,7 @@ struct adm8211_eeprom {
 	u8	lpf_cutoff[14];		/* 0x62 */
 	u8	lnags_threshold[14];	/* 0x70 */
 	__le16	checksum;		/* 0x7E */
-	u8	cis_data[0];		/* 0x80, 384 bytes */
+	u8	cis_data[];		/* 0x80, 384 bytes */
 } __packed;
 
 struct adm8211_priv {