net: fec: use netdev_err_once() instead of netdev_err()

In the case of heavy XDP traffic to be transmitted, the console
will print the error log continuously if there are lack of enough
BDs to accommodate the frames. The log looks like below.

[  160.013112] fec 30be0000.ethernet eth0: NOT enough BD for SG!
[  160.023116] fec 30be0000.ethernet eth0: NOT enough BD for SG!
[  160.028926] fec 30be0000.ethernet eth0: NOT enough BD for SG!
[  160.038946] fec 30be0000.ethernet eth0: NOT enough BD for SG!
[  160.044758] fec 30be0000.ethernet eth0: NOT enough BD for SG!

Not only will this log be replicated and redundant, it will also
degrade XDP performance. So we use netdev_err_once() instead of
netdev_err() now.

Fixes: 6d6b39f1 ("net: fec: add initial XDP support")
Signed-off-by: Wei Fang <wei.fang@nxp.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>