drivers/net/phy/mdio-bitbang.c: Call mdiobus_unregister before mdiobus_free

Based on commit b27393ae

Calling mdiobus_free without calling mdiobus_unregister causes
BUG_ON(). This patch fixes the issue.

The semantic patch that found this issue(http://coccinelle.lip6.fr/

):
// <smpl>
@@
expression E;
@@
  ... when != mdiobus_unregister(E);

+ mdiobus_unregister(E);
  mdiobus_free(E);
// </smpl>

Signed-off-by: Peter Senna Tschudin <peter.senna@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>