[SCSI] megaraid_sas: add the shutdown DCMD cmd to driver shutdown routine

Add the shutdown DCMD cmd to driver shutdown routine to make megaraid sas
FW shutdown proper.

Signed-off-by: Bo Yang <bo.yang@lsi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>