ALSA: timer: Simplify error path in snd_timer_open()

commit 41672c0c upstream.

Just a minor refactoring to use the standard goto for error paths in
snd_timer_open() instead of open code.  The first mutex_lock() is
moved to the beginning of the function to make the code clearer.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
[bwh: Backported to 3.16 as dependency of commit a3933186
 "ALSA: timer: Fix mutex deadlock at releasing card"]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>