V4L/DVB (5175): Pvrusb2: VIDIOC_DBG_[S|G]_REGISTER fixups

Support 64 bit register IDs internally.  Only allow root access to
this API (for both set and get).  Note that actual 64 bit access only
becomes possible once the definition for v4l2_register is updated, but
this change clears the way for it from the viewpoint of the pvrusb2
driver.

Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>