[media] em28xx: move fields wq_trigger and streaming_started from struct...

[media] em28xx: move fields wq_trigger and streaming_started from struct em28xx to struct em28xx_audio

Both wq_trigger and stream_started are used only to control the em28xx
alsa streaming. They don't belong to em28xx common struct.

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