powerpc/mpc5200: Remove obsolete code from mpc5200 MDIO driver

As this driver polls for a complete MDIO transaction, there is no need
to enable interrupts for it.  Furthermore, make both checks for
freeing MDIO-bus irqs consistent.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>