Refactor and clean up broadcast disable.

Some Micrel PHYs have a broadcast-off bit in the Operation Mode Strap
Override register which can be used to disable PHY address 0 as the
broadcast address, so that it can be used as a unique (non-broadcast)
address on a shared bus.

Note that the KSZPHY_OMSO_RMII_OVERRIDE bit is set by default on
KSZ8021/8031.
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>