crypto: mxs-dcp - Check for DMA mapping errors

After calling dma_map_single(), we must also call dma_mapping_error().
This fixes the following warning when compiling with CONFIG_DMA_API_DEBUG:

[  311.241478] WARNING: CPU: 0 PID: 428 at kernel/dma/debug.c:1027 check_unmap+0x79c/0x96c
[  311.249547] DMA-API: mxs-dcp 2280000.crypto: device driver failed to check map error[device address=0x00000000860cb080] [size=32 bytes] [mapped as single]

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>