[SCSI] arcmsr: simplify assumptions in dma_alloc_coherent()

The code currently computes an offset into a dma_alloc_coherent() area
on the assumption that the alignment is imprecise.  In fact, the API
guarantees PAGE_SIZE alignment, so the offset calculation is always
zero: remove it.

[jejb: make description actually descriptive]
Signed-off-by: Tomas henzl <thenzl@redhat.com>
Acked-by: Nick Cheng<nick.cheng@areca.com.tw>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>