[media] cx2341x: move from media/i2c to media/common

The cx2341x module is a helper module for conexant-based MPEG encoders.
It isn't an i2c module at all, instead it should be in common since it is
used by 7 pci and usb drivers to handle the MPEG setup.
It also shouldn't be visible in the config menu as it is always
selected automatically by those drivers that need it.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>