rt2x00: Don't overwrite beacon buffers in pairwise key setup

rt2800 devices use parts of the pariwise key table to store the beacon
frames for beacon 6 and 7. To not overwrite the beacon frame buffers
limit the number of entries we store in the pairwise key table to 222.

Also add some descriptive comments about this shared memory usage.

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>