rt2x00: Make use of sta_add/remove callbacks in rt2800

This allows us to assign a WCID to each STA even for STAs without
crypto key.

To achieve this search for an unused WCID in the HW WCID table and
assign it to the according STA. When configuring a pairwise key for this
STA we don't need to write the MAC address and BSSIDX anymore but just
update the crypto related fields in the WCID_ATTR table.

This has two advantages:

1) Setting a new key for an already available STA (PTK rekeying) is
slightly less expensive and should improve performance in situations
where a lot of rekeying happens (e.g. a huge number of stations and/or
a small rekeying interval)

2) The TXWI now gets a WCID assigned for unencrypted frames which will
be reflected in the corresponding tx status report. This should make tx
status reports in unencrypted AP mode more reliable as we can
distinguish between multiple key-less STAs.

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>