net/phy: enable get_phy_id redefinable

As some PHYs have non-standard PHY ID registers, PHY Id can't
be read correctly by current get_phy_id function, so we enable
get_phy_id redefinable to permit specific PHY driver having own
specific get_phy_id function.

Signed-off-by: Shengzhou Liu <Shengzhou.Liu@freescale.com>