[SCSI] aic79xx: use dma_get_required_mask()

As originally noted by Frederic Temporelli, the aic79xx supports 64
bit addressing, but the initialization code of the driver is wrong: it
tests the available memory size instead of testing the maximum
available memory address.

This patch uses the correct dma_get_required_mask() macros to
determine the correct addressing method.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Cc: Xavier Bru <xavier.bru@bull.net>
CC: Frederic Temporelli <frederic.temporelli@bull.net>
cosmetic fixes
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>