alpha: fix iommu-related boot panic

This fixes a boot panic due to a typo in the recent iommu patchset from
FUJITA Tomonori <tomof@acm.org> - the code used dma_get_max_seg_size()
instead of dma_get_seg_boundary().

It also removes a couple of unnecessary BUG_ON() and ALIGN() macros.

Signed-off-by: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Reported-and-tested-by: Bob Tracy <rct@frus.com>
Acked-by: FUJITA Tomonori <tomof@acm.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>