atm: eni: Add check for dma_map_single

As the potential failure of the dma_map_single(),
it should be better to check it and return error
if fails.

Fixes: 1da177e4

 ("Linux-2.6.12-rc2")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>