[media] tm6000: remove deprecated current_norm

Replace current_norm by g_std. Also initialize the standard to the more
common NTSC-M format (which is also what current_norm used).

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>