phylib: allow incremental scanning of an mii bus

This patch splits the bus scanning code in mdiobus_register() off
into a separate function, and makes this function available for
calling from external code.  This allows incrementally scanning an
mii bus, e.g. as information about which addresses are 'safe' to
scan becomes available.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Acked-by: Andy Fleming <afleming@freescale.com>