[SCSI] 53c700: remove reliance on deprecated cmnd fields

This one's quite tricky.  The 53c700 uses the now deprecated fields of
the command structure for saving the prior command when it does
autosense.  To fix this, we have to add extra fields to the LUN to
accommodate a copy of the real command.

Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>