media: si470x: allow build both USB and I2C at the same time

Currently, either USB or I2C is built. Change it to allow
having both enabled at the same time.

The main reason is that COMPILE_TEST all[yes/mod]builds will
now contain all drivers under drivers/media.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>