ALSA: core: pcm: simplify locking for timers

Fix sparse warning:

sound/core/pcm.c:999:9: warning: context imbalance in
'snd_pcm_detach_substream' - different lock contexts for basic block

There's no real reason to test the same thing twice, and it's simpler
have linear sequences.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200902212133.30964-2-pierre-louis.bossart@linux.intel.com


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