ALSA: aloop: Support runtime change of snd_timer via info interface

ALSA: aloop: Support runtime change of snd_timer via info interface

Show and change sound card timer source with read-write info
file in proc filesystem. Initial string can still be set as
module parameter.

The timer source string value can be changed at any time,
but it is latched by PCM substream open callback (the first one
for a particular cable). At this point it is actually used, that
is the string is parsed, and the timer is looked up and opened.

The timer source is set for a loopback card (the same as initial
setting by module parameter), but every cable uses the value,
current at the moment of open.

Setting the value to empty string switches the timer to jiffies.

Signed-off-by: Andrew Gabbasov <andrew_gabbasov@mentor.com>
Link: https://lore.kernel.org/r/20191120174955.6410-8-andrew_gabbasov@mentor.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>