net: b44: register a fixed phy using fixed_phy_register_100fd if needed

net: b44: register a fixed phy using fixed_phy_register_100fd if needed

In case of bcm47xx a fixed phy is used, which so far is created
by platform code, using fixed_phy_add(). This function has a number of
problems, therefore create a potentially needed fixed phy here, using
fixed_phy_register_100fd.

Due to lack of hardware, this is compile-tested only.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://patch.msgid.link/53e4e74d-a49e-4f37-b970-5543a35041db@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>