[SCSI] megaraid_sas: Fix the frame count calculation

When Driver sent wrong frame count to firmware.  As this particular
command is sent to drive, FW is seeing continuous chip resets and so
the command will timeout.

Signed-off-by Bo Yang<bo.yang@lsi.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>