airo: fix setting zero length WEP key

Patch prevents call set_wep_key() with zero key length. That fix long
standing regression since commit c0380693


"airo: clean up WEP key operations". Additionally print call trace when
someone will try to use improper parameters, and remove key.len = 0
assignment, because it is in not possible code path.

Reported-by: Chris Siebenmann <cks-rhbugzilla@cs.toronto.edu>
Bisected-by: Chris Siebenmann <cks-rhbugzilla@cs.toronto.edu>
Tested-by: Chris Siebenmann <cks@cs.toronto.edu>
Cc: Dan Williams <dcbw@redhat.com>
Cc: <stable@kernel.org>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>