mwifiex: fix ext_capab IE structure definition

EXT_CAPAB_IE format involves IEEE header followed by bytestream of
capabilities. Current structure has incorrect member u8 for data;
fix it by defining it as u8[0].

Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>