[media] mn88472: rename mn88472_c.c => mn88472.c

Original plan was to implement driver as one file per used demod
standard (mn88472_c.c, mn88472_t.c and mn88472_t2.c). However, that
plan was a mistake as driver code differences are so small between
different standards. Due to that rename this file and implement all
the needed functionality to that file.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>