airo: Fix integer overflow warning

On BigEndian gcc complains:

drivers/net/wireless/airo.c: In function ‘sniffing_mode’:
drivers/net/wireless/airo.c:4809: warning: integer overflow in expression

Fix this by doing the bitwise AND on the host-endian value.

Signed-off-by: John W. Linville <linville@tuxdriver.com>