net: fix build failure in xilinx

Commit 71c6c837 (drivers/net: fix tasklet misuse issue) introduced a
build failure in the xilinx driver. axienet_dma_err_handler isn't
declared before its use in axienet_open.

This patch provides the prototype before axienet_open.

Cc: Xiaotian Feng <dannyfeng@tencent.com>
Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: David S. Miller <davem@davemloft.net>