[media] v4l: atmel-isi: remove SOF wait in start_streaming()

when a userspace applications calls the VIDIOC_STREAMON ioctl. The
V4L2 core calls the soc_camera_streamon function, which is responsible
for starting the video stream. It does so by first starting the atmel-isi
host by a call to the vb2_streamon function, and then starting the sensor
by a call to the video.s_stream sensor subdev operation.
That means we wait for a SOF in start_streaming() before call sensor's
s_stream(). It is possible no VSYNC interrupt arrive as the sensor
hasn't been started yet.
To avoid such case, this patch remove the code to wait for the VSYNC
interrupt. And such code is not necessary.

Reported-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>