uapi: wireless: Replace zero-length array with flexible-array member

Zero-length and one-element arrays are deprecated, and we are moving
towards adopting C99 flexible-array members, instead.

Address the following warnings seen under GCC-13 and
-fstrict-flex-arrays=3 enabled:
drivers/staging/ks7010/ks_wlan_net.c:1597:50: warning: array subscript 0 is outside array bounds of ‘__u8[0]’ {aka ‘unsigned char[]’} [-Warray-bounds=]
drivers/staging/ks7010/ks_wlan_net.c:1603:61: warning: array subscript 16 is outside array bounds of ‘__u8[0]’ {aka ‘unsigned char[]’} [-Warray-bounds=]
drivers/staging/ks7010/ks_wlan_net.c:1604:61: warning: array subscript 24 is outside array bounds of ‘__u8[0]’ {aka ‘unsigned char[]’} [-Warray-bounds=]
drivers/staging/ks7010/ks_wlan_net.c:1600:61: warning: array subscript 16 is outside array bounds of ‘__u8[0]’ {aka ‘unsigned char[]’} [-Warray-bounds=]
drivers/staging/ks7010/ks_wlan_net.c:1586:50: warning: array subscript 0 is outside array bounds of ‘__u8[0]’ {aka ‘unsigned char[]’} [-Warray-bounds=]

This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
routines on memcpy() and help us make progress towards globally
enabling -fstrict-flex-arrays=3 [1].

This results in no differences in binary output.

Link: https://github.com/KSPP/linux/issues/21
Link: https://github.com/KSPP/linux/issues/261
Link: https://gcc.gnu.org/pipermail/gcc-patches/2022-October/602902.html [1]
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>