net: dpaa2-mac: fix the case with no 'phys' in the DT node

commit  867b5c9b17c11911281ab68db3d91cc141f5fa20 from
https://github.com/nxp-imx/linux-imx

 lf-6.1.y

In case there are no 'phys' described in the DT node, the driver should
not error out. Instead we should just mark down that there are no SerDes
phys which in turn will determine the interface to not support dynamic
protocol reconfiguration.

Fixes: b19b8a126a7d ("net: dpaa2-mac: extend serdes code for multiple phys")
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: Xiaolei Wang <xiaolei.wang@windriver.com>