[SCSI] mvsas: remove unused variable in mvs_task_exec()

We don't use "dev" any more after 07ec747a5f ("libsas: remove
ata_port.lock management duties from lldds") and it causes a compile
warning.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Xiangliang Yu <yuxiangl@marvell.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>