ALSA: ASoC: Convert DAPM pop time configuration to debugfs

Pop time configuration is a debugging feature which shouldn't be used in
normal operation so move it to debugfs where it is clearer. This also
simplifies the code since debugfs provides standard facilities for
exposing simple integer values to user space.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>