[ARM] Remove definition of MAX_DMA_CHANNELS to zero

Since we now only build arch/arm/kernel/dma.c on machine types
which set ISA_DMA_API, we don't need to define MAX_DMA_CHANNELS
to 0 to indicate this - this definition becomes superfluous.
Remove it.

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