netdev/phy: Make get_phy_id() static and quit EXPORTing it.

This function is only referenced from within phy_device.c, so there is
no reason to export it.  In fact, we can make it static.

Signed-off-by: David Daney <david.daney@cavium.com>
Acked-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>