mac802154: fix destructon ordering for ieee802154 devices

mutex_destroy() must be called before wpan_phy_free(), because it puts the last
reference and frees memory. Catched as overwritten poison in kmalloc-2048.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Cc: Alexander Smirnov <alex.bluesman.smirnov@gmail.com>
Cc: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: linux-zigbee-devel@lists.sourceforge.net
Cc: netdev@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>