ALSA: control: Provide a helper to look for the preferred subdevice

Instead of open-coding the search over the control file loop, provide
a helper function for the preferred subdevice assigned to the current
process.

Reviewed-by: Jaroslav Kysela <perex@perex.cz>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --cc include/sound/control.h
Simple merge

diff --cc sound/core/control.c
Simple merge

diff --cc sound/core/pcm.c
Simple merge

diff --cc sound/core/rawmidi.c
Simple merge