ALSA: timer: Code cleanup

commit c3b1681375dc6e71d89a3ae00cc3ce9e775a8917 upstream.

This is a minor code cleanup without any functional changes:
- Kill keep_flag argument from _snd_timer_stop(), as all callers pass
  only it false.
- Remove redundant NULL check in _snd_timer_stop().

Signed-off-by: Takashi Iwai <tiwai@suse.de>
[bwh: Backported to 3.2: adjust to apply after previously backported
 "ALSA: timer: Fix race between stop and interrupt"]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>