vfio: mdev: make a couple of functions and structure vfio_mdev_driver static

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: Xiongwei Song <sxwjean@gmail.com>
Reviewed-by: Quan Xu <quan.xu0@gmail.com>
Reviewed-by: Liu, Yi L <yi.l.liu@intel.com>
Reviewed-by: Kirti Wankhede <kwankhede@nvidia.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>