fixups for the dma_set_mask beahvior change in 5.1

[Christoph Hellwig]

Hi all,

in 5.1 I fixed up the DMA mapping API to always accept larger than
required DMA mask.  Except that I forgot about a check in the arm
code that is not required any more, and about the case where a
architecture only supports a 32-bit dma mask, but could potentially
generate large addresses due to offsets for the DMA address.

These two patches should fix up these issues (which were only found by
code inspection).