media: stv6110x: Implement probe/remove for stv6110x

Refactor out the common parts of stv6110x_probe() and stv6110x_attach()
into separate functions.

This provides the needed functionality to use dvb_module_probe() instead
of dvb_attach()!

Signed-off-by: Tobias Klausmann <tobias.johannes.klausmann@mni.thm.de>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>