crypto: atmel-aes - Retire dma_request_slave_channel_compat()

The driver no longer boots in legacy mode, only via DT. This makes the
dma_request_slave_channel_compat() redundant.
If ever the filter function would be executed it will return false as the
dma_slave is not really initialized.

Switch to use dma_request_chan() which would allow legacy boot if ever
needed again by configuring dma_slave_map for the DMA driver.

At the same time skip allocating memory for dma_slave as it is not used
anymore.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>