ethernet: micrel: fix some error codes

There were two issues here:
1) dma_mapping_error() return true/false but we want to return -ENOMEM
2) If dmaengine_prep_slave_sg() failed then "err" wasn't set but
   presumably that should be -ENOMEM as well.

I changed the success path to "return 0;" instead of "return ret;" for
clarity.

Fixes: 94fe8c68 ('ks8842: Support DMA when accessed via timberdale')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>