dpaa2-mac: workaround for zero advertising on the MAC

commit ead765a527718bfb3b1cb6c673aa882b65afd095 from
git://source.codeaurora.org/external/qoriq/qoriq-components/linux

It seems there are some circumstances in the latest MC firmware versions
where an empty advertising field is returned by the dpmac_get_link_cfg()
command. This will eventually lead to the link not going up.
Work around this issue by skipping PHY reconfiguration if this happens.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: Meng Li <Meng.Li@windriver.com>