fakelb: fix schedule while atomic

This patch changes the spinlock to mutex for the available fakelb phy
list. When holding the spinlock the ieee802154_unregister_hw is called
which holding the rtnl_mutex, in that case we get a "BUG: sleeping function
called from invalid context" error. We simple change the spinlock to
mutex which allows to hold the rtnl lock there.

Signed-off-by: Alexander Aring <aar@pengutronix.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>