rt2x00: Fix rt2800 key assignment in multi bssid setups

When setting up multiple BSSIDs in AP mode on an rt2800pci device we
previously used the STAs AID to select an appropriate key slot. But
since the AID is per VIF we can end up with two STAs having the same AID
and thus using the same key index. This resulted in one STA overwriting
the key information of another STA.

Fix this by simply searching for the next unused entry in the pairwise
key table.

Also bring the key table init in sync with deleting keys by initializing
the key table entries to 0 instead of 1.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>