fsl_pq_mdio: Fix compile failure

Add EXPORT_SYMBOL_GPL(fsl_pq_mdio_bus_name) for module builds

Signed-off-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: David S. Miller <davem@davemloft.net>