dmaengine: Remove net_dma_find_channel

Since commit 7bced397 ("net_dma: simple removal") removed the net_dma
support entirely, net_dma_find_channel has no users left. Remove the function
entirely.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>