media: dvb: init i2c already in it930x_frontend_attach

i2c bus is already needed when the frontend is probed, so init it already
in it930x_frontend_attach. That prevents errors like:

si2168: probe of 6-0067 failed with error -5

Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Signed-off-by: Jose Alberto Reguero <jose.alberto.reguero@gmail.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>