dmaengine: bcm2835: Fix dma driver for BCM2835-38

The previous commit broke support on older devices.
Make the breaking parts of patch conditional on
the device being used.

Fixes: 6e1856ac7c39 ("dmaengine: bcm2835: HACK: Support DMA-Lite channels")

Signed-off-by: Dom Cobley <popcornmix@gmail.com>