[SCSI] mvsas: check hd whether unplugged

if unplugged, driver's queuecommand function will return SAS_PHY_DOWN.
task->lldd_task is used for saving its slot info.

Signed-off-by: Ke Wei <kewei@marvell.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>