ALSA: seq: Avoid open-code for getting timer resolution

Instead of open-coding for getting the timer resolution, use the
standard snd_timer_resolution() helper.

The original code falls back to the callback function when the
resolution is zero, but it must be always so when the callback
function is defined.  So this should be no functional change.

Signed-off-by: Takashi Iwai <tiwai@suse.de>