uio: make symbol 'uio_class_registered' static

Fixes the following sparse warning:

drivers/uio/uio.c:277:6: warning:
 symbol 'uio_class_registered' was not declared. Should it be static?

Fixes: ae61cf5b ("uio: ensure class is registered before devices")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>