ath9k: Remove ATH9K_BEACON_PERIOD mask

Earlier beacon_interval is used to hold interval value and
some flags (ATH9K_BEACON_ENA &ATH9K_BEACON_PERIOD). So to
extract interval ATH9K_BEACON_PERIOD is used. Those flags
were completely removed. So masking beacon_interval is
not required.

Signed-off-by: Rajkumar Manoharan <rmanoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>