aic79xx: use BIOS settings

This patch fixes the aic79xx driver to properly respond to BIOS
settings.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>
Signed-off-by: Adrian Bunk <bunk@stusta.de>