ARM: dma-mapping: allow larger DMA mask than supported

Since the Linux 5.1 merge window we allow drivers to just set the
largest DMA mask they support instead of falling back to smaller ones.

But I forgot to remove a check that prohibits this behavior in the
arm DMA code, as it is rather hidden.  There is not reason for this check
as the code will do the right thing for a "too large" DMA mask, so
just remove it.

Fixes: 9eb9e96e ("Documentation/DMA-API-HOWTO: update dma_mask sections")
Signed-off-by: Christoph Hellwig <hch@lst.de>