media: videobuf-dma-sg: void function return statements are not generally useful

void function videobuf_vm_close return statements are not generally useful,
so deleted the return in function videobuf_vm_close().

Signed-off-by: lijian <lijian@yulong.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>