Move common code into a new function.

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

[mchehab+samsung@kernel.org: fix an out of order error return code]
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>