ath5k: initialize calibration timers

Initialize calibration timers on reset, since otherwise they might be in the
future and the calibration tasklet might not be scheduled for a long time.

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