The functions vfio_mdev_probe, vfio_mdev_remove and the structure
vfio_mdev_driver are only used in this file, so make them static.

Clean up sparse warnings:
drivers/vfio/mdev/vfio_mdev.c:114:5: warning: no previous prototype
for 'vfio_mdev_probe' [-Wmissing-prototypes]
drivers/vfio/mdev/vfio_mdev.c:121:6: warning: no previous prototype
for 'vfio_mdev_remove' [-Wmissing-prototypes]
Signed-off-by: NXiongwei Song <sxwjean@gmail.com>
Reviewed-by: NQuan Xu <quan.xu0@gmail.com>
Reviewed-by: NLiu, Yi L <yi.l.liu@intel.com>
Reviewed-by: NKirti Wankhede <kwankhede@nvidia.com>
Signed-off-by: NAlex Williamson <alex.williamson@redhat.com>