[media] mn88473: calculate the IF register values

Add xtal as a configuration parameter so it can be used
in the IF register value calculation. If not set in the
configuration then use a default value.

Signed-off-by: Benjamin Larsson <benjamin@southpole.se>
Reviewed-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>