airo: fix transmit_802_11_packet()

a) gaplen would better be stored little-endian
b) for control packets (shorter than 24-byte header) we ended up with
        bap_write(ai, hdrlen == 30 ?
                (const u16*)&gap.gaplen : (const u16*)&gap, 38 - hdrlen, BAP1);
passing to card the data past the end of gap (i.e. random stuff from stack)
and did _not_ feed the gaplen at the right offset.
c) sending the contents of uninitialized fields of struct is Not Nice(tm) either

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