net/sonic: Use dma_mapping_error()

With CONFIG_DMA_API_DEBUG=y, calling sonic_open() produces the
message, "DMA-API: device driver failed to check map error".
Add the missing dma_mapping_error() call.

Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Acked-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: David S. Miller <davem@davemloft.net>