media: ddbridge: fix sparse warnings

Fix several

  drivers/media/pci/ddbridge/ddbridge-core.c: warning: symbol ... was not declared. Should it be static?
  drivers/media/pci/ddbridge/ddbridge-core.c: warning: Using plain integer as NULL pointer
  drivers/media/pci/ddbridge/ddbridge-io.h: warning: cast removes address space of expression
  drivers/media/pci/ddbridge/ddbridge-io.h: warning: incorrect type in argument 1 (different address spaces)

at multiple places.

Cc: Ralph Metzler <rjkm@metzlerbros.de>
Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>