[SCSI] mpt2sas : fix oops when firmware sends large sense buffer size

There is a bug in firmware where the reply message frame says there is a
16kb sense buffer, when in reality its only 20 bytes.  This fix insures
the memcpy action doesn't corrupte the memory beyond the 90 bytes allocated in
the scsi command for sense buffer.

Signed-off-by: Eric Moore <eric.moore@lsi.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>