net: bcmgenet: always select FIXED_PHY

There is no need to do the following:

select FIXED_PHY if BCMGENET=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: b0ba512e225d ("net: bcmgenet: enable driver to work without device tree"
Fixes: bdaa53bd ("net: bcmgenet: hook into the build system")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>