ath5k: fix TSF reset

to reset the TSF, AR5K_BEACON_RESET_TSF has to be 1, not 0. also we have a
function for that so use it.

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