[media] ml86v7667: implement g_std() method

The driver was written with the 'soc_camera' use in mind, however the g_std()
video method was forgotten. Implement it at last...

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>