To ensure that phy_mask and any future elements of the mii_bus
structure are initialized use kzalloc() instead of kmalloc().

This fixes an issue in which phy_mask was not being initialized
and we would skip random phy addresses when scanning.
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>