ALSA: timer: Unify timer callback process code

The timer core has two almost identical code for processing callbacks:
once in snd_timer_interrupt() for fast callbacks and another in
snd_timer_tasklet() for delayed callbacks.  Let's unify them.

In the new version, the resolution is read from ti->resolution at each
call, and this must be fine; ti->resolution is set in the preparation
step in snd_timer_interrupt().

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