vfio-mdev/samples: make some symbols static

Make some structs and functions static to fix build warning, parts of
warning shown below,

samples/vfio-mdev/mtty.c:730:5: warning: symbol 'mtty_create' was not declared. Should it be static?
samples/vfio-mdev/mtty.c:780:5: warning: symbol 'mtty_remove' was not declared. Should it be static?
samples/vfio-mdev/mtty.c:802:5: warning: symbol 'mtty_reset' was not declared. Should it be static?
samples/vfio-mdev/mtty.c:818:9: warning: symbol 'mtty_read' was not declared. Should it be static?
samples/vfio-mdev/mtty.c:877:9: warning: symbol 'mtty_write' was not declared. Should it be static?
samples/vfio-mdev/mtty.c:1070:5: warning: symbol 'mtty_get_region_info' was not declared. Should it be static?
samples/vfio-mdev/mtty.c:1119:5: warning: symbol 'mtty_get_irq_info' was not declared. Should it be static?
samples/vfio-mdev/mtty.c:1143:5: warning: symbol 'mtty_get_device_info' was not declared. Should it be static?
samples/vfio-mdev/mtty.c:1275:5: warning: symbol 'mtty_open' was not declared. Should it be static?
samples/vfio-mdev/mtty.c:1281:6: warning: symbol 'mtty_close' was not declared. Should it be static?
samples/vfio-mdev/mtty.c:1305:30: warning: symbol 'mtty_dev_groups' was not declared. Should it be static?

Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Kirti Wankhede <kwankhede@nvidia.com>
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
[aw: wrap long lines]
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>