ath5k: Increase "fudge" for beacon timers

We use FUDGE to make sure the next TBTT is ahead of the current TU.
Since we later substract AR5K_TUNE_SW_BEACON_RESP (10) in the timer
configuration we need to make sure it is bigger than that.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>