dma: pl330: Use dma_get_slave_channel() in the of xlate callback

Currently the driver uses dma_request_channel() with a custom filter function to
find the requested channel. This will loop over all available channels until the
one we want has been found, but we already know which channel we want to
request, so we can dma_get_slave_channel(). This also makes the code a bit
shorter cleaner.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>