net: dsa: Fix NULL checking in dsa_slave_set_eee()

This function can't succeed if dp->pl is NULL.  It will Oops inside the
call to return phylink_ethtool_get_eee(dp->pl, e);

Fixes: 1be52e97

 ("dsa: slave: eee: Allow ports to use phylink")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>