staging: most: configfs: Make mdev_link_list static

Fix sparse warning:

drivers/staging/most/configfs.c:34:18: warning:
 symbol 'mdev_link_list' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>