ath9k: Remove redundant beacon_interval

The variable appears in both ath_softc and ath_beacon_config.
The struct ath_beacon_config is embedded in ath_softc. The redundant
variable was added by commit id 57c4d7b4.

Signed-off-by: Steve Brown <sbrown@cortland.com>
Reviewed-by: Mohammed Shafi <shafi.ath9k@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>