sh: Fix up MAX_DMA_CHANNELS definition when DMA is disabled.

MAX_DMA_CHANNELS is tested for the total number of channels in order to
populate an IRQ map. Stub this out completely when no DMA support is
enabled -- as used to be the default behaviour before this was
generalized for use by the dmaengine code.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>