[media] adv7842: return 0 if no change in s_dv_timings

Return 0 if the new timings are equal to the current timings as
it caused extra cp-loss/lock interrupts.

Signed-off-by: Martin Bugge <marbugge@cisco.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>