net: systemport: always select FIXED_PHY

There is no need to do the following:

select FIXED_PHY if SYSTEMPORT=y, as this implies that we will not be able
to build and/or run the driver correctly when built as a module, which
is no longer an issue since commit 37e9a690 ("net: phy: export
fixed_phy_register()")

Fixes: a3862db2 ("net: systemport: hook SYSTEMPORT driver in the build")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>