[SCSI] add missing hold_mcs parameter to the spi transport class

This parameter is important only to people who take the time to tune the
margin control settings, otherwise it's completely irrelevant.  However,
just in case anyone should want to do this, it's appropriate to include
the parameter.

I don't do anything with it in DV by design, so the parameter will come
up as off by default, so if anyone actually wants to play with the
margin control settings they'll have to enable it under the
spi_transport class first.

I also updated the transfer settings display to report all of the PPR
settings instead of only DT, IU and QAS

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