[media] cx25840: initialize the standard to NTSC_M

This is necessary since the *_std_setup functions rely on a valid state->std
field.

Also fix the cx23888_std_setup() to test for 60Hz instead of NTSC, just like
cx25840_std_setup() does.

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