V4L/DVB (10519): em28xx: Fix for em28xx audio startup

Essentially if a snd_em28xx_capture_trigger() stop followed by a snd_em28xx_capture_trigger() start would not yield any
data because there was some logic put in with an adev->shutdown variable which did not seem warranted in my humble opinion.
It would cause snd_em28xx_capture_trigger start never to start up the audio stream until the device was closed and
reopened again. Upon re-opening the device adev->shutdown is reset and audio data would again flow.

Signed-off-by: Robert Krakora <rob.krakora@messagenetsystems.com>
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>