p54: utilize all available key slots for decryption offload

This patch takes care of outstanding TODOs:
/* TODO: some devices have 4 more free slots for rx keys */

Now the driver can utilize all available key slots instead of just 4.
Obviously, this helps most in AP/IBSS(/MESH) mode, when
we have to use more different keys.

Signed-off-by: Christian Lamparter <chunkeey@web.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>