net: dwc-xlgmac: fix an error code in xlgmac_alloc_pages()

The dma_mapping_error() returns true if there is an error but we want
to return -ENOMEM and not 1.

Fixes: 65e0ace2

 ("net: dwc-xlgmac: Initial driver for DesignWare Enterprise Ethernet")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Jie Deng <jiedeng@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>