[media] mxl5005s: fix compiler warning

Removed the unused Xtal_Int variable. That made it also possible to remove a
related function. However, the code of that function has been preserved in a
comment describing an equation. Without that function that comment would
have been hard to understand.

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