ALSA: timer: Code cleanup

ALSA: timer: Code cleanup

Orabug: 23330803

[ Upstream commit c3b1681375dc6e71d89a3ae00cc3ce9e775a8917 ]

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>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
(cherry picked from commit aef523ec3812f822fdd256f64c0d941a0588647f)

Signed-off-by: Dan Duval <dan.duval@oracle.com>