um: make PCI emulation driver init/exit static

The functions aren't used elsewhere, so they can be static.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 68f5d3f3 ("um: add PCI over virtio emulation driver")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-By: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>