[ARM] dma: correct dma_supported() implementation

dma_supported() is supposed to indicate whether the system can support
the DMA mask it was passed, which depends on the maximal address which
can be returned for DMA allocations.  If the mask is smaller than that,
we are unable to guarantee that the driver can reliably obtain suitable
memory.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>