V4L/DVB (4930): Usbvision_v4l2 : mmap corrected to get all frames

- private ioctls UVIOCSREG/UVIOCGREG translated to the
	VIDIOC_INT_G_REGISTER/VIDIOC_INT_S_REGISTER
- lost frame bug corrected (mmap rework)
- reset scratch buffer is no buffer is queued (prevents useless scratch
	overflow management)

Signed-off-by: Thierry MERLE <thierry.merle@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>