[media] it913x: rename its tuner driver to tuner_it913x

There are three drivers with *it913x name on it, and they all
belong to the same device:
	a tuner, at it913x.c;
	a frontend: it913x-fe.c;
	a bridge: it913x.c, renamed to dvb_usb_it913x by the
building system.

This is confusing. Even more confusing are the two .c files with
the same name under different directories, with different contents
and different functions. So, prepend the tuner one.

This also breaks the out-of-tree compilation system.

Reported-by: Frederic Fays <frederic.fays@gmail.com>
Acked-by: Antti Palosaari <crope@iki.fi>
Reviewed-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>