media: ddbridge: move ddb_unmap(), cleanup modparams

adapter_alloc is only used from within ddbridge-core, so move it there,
this removes the need for prototyping/referencing the variable. While at
it, msi isn't needed outside of ddbridge-main, so don't extref that one
aswell.

Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>