net: dsa: bcm_sf2: always select FIXED_PHY

There is no need to do the following:

select FIXED_PHY if NET_DSA_BCM_SF2=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 37e9a6904520 ("net: phy: export
fixed_phy_register()").

Fixes: 246d7f773c13ca ("net: dsa: add Broadcom SF2 switch driver")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>