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: NWei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/uio/uio.c

@@ -274,7 +274,7 @@ static struct class uio_class = {
 	.dev_groups = uio_groups,
 };
 
-bool uio_class_registered;
+static bool uio_class_registered;
 
 /*
  * device functions