airo: bug in airo_interrupt() handling on incoming 802.11

On big-endian we end up with swapped first two bytes in packet,
due to earlier conversion to host-endian and forgotten conversion
back.

The code we calculated that host-endian for had been duplicated
several time - it finds the 802.11 MAC header length by the first
two bytes of packet; taken into a new helper (header_len(__le16 ctl)).

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: John W. Linville <linville@tuxdriver.com>