crypto: omap-aes - Convert to dma_request_slave_channel_compat()

Use the dma_request_slave_channel_compat() call instead of
the dma_request_channel() call to request a DMA channel.
This allows the omap-aes driver use different DMA engines.

CC: Dmitry Kasatkin <dmitry.kasatkin@intel.com>
Signed-off-by: Mark A. Greer <mgreer@animalcreek.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>