rt2500usb: disallow to set WEP key with non zero index

On our hardware (050d:7050 Belkin Components F5D7050 Wireless G Adapter),
setting any WEP key with non zero index, cause rx frames corruption.

Note: perhaps (I did not check) this can be fixed differently - by using
hw_key_idx the same as true MAC key index. But according to the comment in
rt2x00mac_set_key():

"the hardware requires keys to be assigned in correct order (When key 1
is provided but key 0 is not, then the key is not found by the hardware
during RX)"

this will be quite problematic. Since WEP should not be used, disabling
hardware crypto offload for it will not hurt much. Beside static
one key WEP will still be offloaded.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>