[media] it913x: rename tuner_it913x => it913x

Remove tuner_ prefix from module name and file names. Prefix was
added due to file name conflict on media out-tree build system.
Demodulator having same name does not exists anymore. So lets
remove dumb prefix.

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