scsi: sd: make sd_devt_release() static

Fixes the following sparse warning:

drivers/scsi/sd.c:3087:6: warning:
 symbol 'sd_devt_release' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>