[media] pwc: Avoid sending mode info to the camera when it is not needed

Since we always do a set_video_mode on stream start, there is no need
to actually send the mode info to the device on a s_fmt / s_parm ioctl.
Not doing this saves us doing (slow) usb io.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>