[SCSI] aic7xxx: move to dma_get_required_mask() and correct 39 bit assumptions

This patch moves aic7xxx over to the dma_get_required_mask() API and
dumps its open coded memory check.

It also appears from this bug:

https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=167049

That 39 bit addressing doesn't work on older cards.  I surmise that the
AHC_LARGE_SCBS flag is the one that marks cards capable of using 39 bit
addressing, so I also folded that check into the code.

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