ath5k: Always set IFS intervals on reset

 * Make sure we always set IFS timings even if no
 coverage class is set. If we don't we'll miss the
 needed changes for different bwmodes.

Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>

Signed-off-by: John W. Linville <linville@tuxdriver.com>