scsi: sd: Remove a superfluous assignment

Since blk_rq_bytes(req) returns req->__data_len, assigning that value to
req->__data_len is superfluous. Hence remove that assignment.

See also commit 5db44863 ("[SCSI] sd: Implement support for WRITE SAME").

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>