V4L/DVB (10558): bttv: norm value should be unsigned

The norm value in the driver is an index into an array and the the driver
doesn't allow it to be negative or otherwise invalid.  It should be
unsigned but wasn't in all places.

Fix some structs and functions to have the norm be unsigned.  Get rid of
useless checks for "< 0".  Most of the driver code can't handle a norm
value that's out of range, so change some ">= BTTV_TVNORMS" checks to
BUG_ON().  There's no point in silently ignoring invalid driver state just
to crash because of it later.

Reported-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Trent Piepho <xyzzy@speakeasy.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>