media: dvb-core: add helper functions for I2C binding

The dvb_attach()/dvb_detach() methods are ugly hacks designed
to keep using the I2C low-level API. The proper way is to
do I2C bus bindings instead.

Several modules were already converted to use it. Yet,
it is painful to use it, as lots of code need to be
duplicated.

Make it easier by providing two new helper functions:
	- dvb_module_probe()
	- dvb_module_release()

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