media: usbtv: Use the constant for supported standards

Use the USBTV_TV_STD define instead of repeating ourselves.

Signed-off-by: Hugo Grostabussiat <bonstra@bonstra.fr.eu.org>
Signed-off-by: Hans Verkuil <hansverk@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>