V4L/DVB (9031): uvcvideo: Fix incomplete frame drop when switching to a variable size format.

When streaming in a fixed size format the driver sets a flag in the uvc_queue
structure to drop incomplete incoming frames. The flag wasn't cleared when
switching to a variable size format, which resulted in a broken
'MJPEG after YUV'.

Signed-off-by: Laurent Pinchart <laurent.pinchart@skynet.be>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>