dmaengine: mmp_pdma: Do not warn when IRQ is shared by all chans

dmaengine: mmp_pdma: Do not warn when IRQ is shared by all chans

When there's a single interrupt for all the DMA channels, the
unsuccessful attempt to request separate IRQs emits useless warnings:

  [    1.370381] mmp-pdma d4000000.dma: IRQ index 1 not found
  ...
  [    1.412398] mmp-pdma d4000000.dma: IRQ index 15 not found
  [    1.418308] mmp-pdma d4000000.dma: initialized 16 channels

Avoid that, treating the IRQs as optional.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Link: https://lore.kernel.org/r/20200601192337.172869-1-lkundrak@v3.sk
Signed-off-by: Vinod Koul <vkoul@kernel.org>