ARM: dma-api: fix off-by-one error in __dma_supported()

When validating the mask against the amount of memory we have available
(so that we can trap 32-bit DMA addresses with >32-bits memory), we had
not taken account of the fact that max_pfn is the maximum PFN number
plus one that would be in the system.

There are several references in the code which bear this out:

mm/page_owner.c:
	for (; pfn < max_pfn; pfn++) {
	}

arch/x86/kernel/setup.c:
	high_memory = (void *)__va(max_pfn * PAGE_SIZE - 1)

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