[SCSI] aic79xx: update to use scsi_transport_spi

This patch updates the aic79xx driver to take advantage of the
scsi_transport_spi infrastructure. Patch is quite a mess as some
procedures have been reshuffled to be closer to the aic7xxx driver.

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