at76c50x-usb: Make WEP encryption working.

Currently the driver uses HW encryption.
Whenever mac80211 calls the set_key() callback the driver restarts the
whole HW configuration procedure, in order to set (also) the new
WEP key.
However, by doing this, it causes the card to loose association information,
and the HW becomes unable to communicate with the BSS.

This patch adds support for sending another HW command, that sets only
the wep key, instead of resetting all.
Mac80211 key-set requests are thus handled via this new command.

Tested on my at76c503

Signed-off-by: John W. Linville <linville@tuxdriver.com>