[SCSI] aic79xx: make driver respect nvram for IU and QAS settings

This patch allows the Adaptec firmware to pass on its values for Packetize and
QAS.  To do this, the settings max_iu and max_qas have been introduced into
the SPI transport class and populated from the adaptec NVram tables.  Domain
validation in the SPI transport class will respect the max settings when
configuring to the highest possible speed for testing.

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