media: ddbridge: move ddb_wq and the wq+class initialisation to -core

Move the ddbridge module initialisation and cleanup code to ddbridge-core
and set up the ddb_wq workqueue there, and create and destroy the ddb
device class there aswell. Due to this, the prototypes for ddb_wq,
ddb_class_create() and ddb_class_destroy() aren't required in ddbridge.h
anymore, so remove them. Also, declare ddb_wq and the ddb_class_*()
functions static.

Picked up from the upstream dddvb-0.9.33 release.

Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>