wifi: mt76: mt7925: replace zero-length array with flexible-array member

wifi: mt76: mt7925: 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],
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, which is beneficial
to cultivate a high-quality code.[2]

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Zhang Kunbo <zhangkunbo@huawei.com>
Link: https://patch.msgid.link/20241218074552.3271542-1-zhangkunbo@huawei.com
Signed-off-by: Felix Fietkau <nbd@nbd.name>