net: dsa: add includes for ethtool and phy_fixed definitions

net/dsa/slave.c uses functions and structures declared in phy_fixed.h
but does not explicitely include it, while dsa.h needs structure
declarations for 'struct ethtool_wolinfo' and 'struct ethtool_eee', fix
those by including the correct header files.

Fixes: ec9436ba ("net: dsa: allow drivers to do link adjustment")
Fixes: ce31b31c ("net: dsa: allow updating fixed PHY link information")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>