[media] cx25840: relax a Vsrc check

The cx23888 reports a slightly different Vsrc value than the other
chip variants do. Relax the check by 1, otherwise cx25840_set_fmt()
would fail for the cx23888.

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