net: calxedaxgmac: fix rx DMA mapping API size mismatches

Fix the mismatch in the DMA mapping and unmapping sizes for receive. The
unmap size must be equal to the map size and should not be the actual
received frame length. The map size should also be adjusted by the
NET_IP_ALIGN size since the h/w buffer size (dma_buf_sz) includes this
offset.

Also, add a missing dma_mapping_error check in xgmac_rx_refill.

Reported-by: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: David S. Miller <davem@davemloft.net>