V4L/DVB (6216): V4L: Int if: add vidioc_int_g_ifparm, other updates

vidioc_int_g_ifparm can be used to obtain hardware-specific information
about the interface used by the slave.

Rearrange v4l2-int-device.h as well.

Also remove useless & characters.

Signed-off-by: Sakari Ailus <sakari.ailus@nokia.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>