[PATCH] libertas: remove adapter->beaconperiod

beaconperiod was initialized with MRVDRV_BEACON_INTERVAL, but there is
no code that would ever change it's value. We can use the define directly.

Signed-off-by: Holger Schurig <hs4233@mail.mn-solutions.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>