phylib: do EXPORT_SYMBOL on get_phy_id

Commit cac1f3c8

 factored out the code for get_phy_id so that it
could be reused in multiple places.  Turns out that some of the
users can be modular, so we need to export this symbol as well.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>