rt2x00: rt2800lib: pass beacon index to rt2800_clear_beacon_register

Instead of precomputing the beacon base in
each caller, pass the beacon index to the
'rt2800_clear_beacon_register' function
and compute the beacon base in there.

This allows to simplify the caller functions
a bit.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>