[libata] remove SCSI host lock and serial number usage from ata_scsi_queuecmd

cmd->serial_number is never tested in any path we reach; therefore we may
remove the call to scsi_cmd_get_serial() inside DEF_SCSI_QCMD, the SCSI
host_lock acquisition surrounding it, and our own SCSI host_lock
unlock+relock cycle.

Signed-off-by: Jeff Garzik <jgarzik@redhat.com>