net/phy: fixup for get_phy_id

commit 3c6928fd "net: phy: fix warnings with W=1" caused
some PHYs(e.g. CS4315/CS4340) not working. This patch fixes the
warning and make those special PHYs working as well.

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