airo: clean up WEP key operations

get_wep_key() and set_wep_key() combind both get/set of the actual WEP
key and get/set of the transmit index into the same functions.  Split those
out so it's clearer what is going one where.  Add error checking to WEP
key hardware operations too.

Signed-off-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>