ALSA: timer: Fix the breakage of slave link open

A silly mistake was made while applying the fix for potential races in
commit 6a34367e ("ALSA: timer: Fix possible race at assigning a
timer instance"): when a slave PCM is opened and succeeds, it doesn't
return but proceeds to the master timer open code instead.  Plug the
hole and beautify a bit.

Fixes: 6a34367e ("ALSA: timer: Fix possible race at assigning a timer instance")
Reported-by:  <syzbot+4476917c053f60112c99@syzkaller.appspotmail.com>
Link: https://lore.kernel.org/r/20191111173642.6093-1-tiwai@suse.de


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