mac802154: Avoid rtnl deadlock in mac802154_wpan_ioctl().

->ndo_do_ioctl() can be entered with the rtnl lock already held,
for example when sending a wext ioctl to a device (in which case
the rtnl lock is taken by wext_ioctl_dispatch()), but
mac802154_wpan_ioctl() currently unconditionally takes the rtnl
lock on entry, which can cause deadlocks.

To fix this, bail out of mac802154_wpan_ioctl() before taking the
rtnl lock if the ioctl cmd is not one of the cmds we implement.

Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>
Acked-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>