[media] em28xx-audio: make sure audio is unmuted on open()

In all cases, when the first capture is called, we need to
call the code that unmutes the volume.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>