drivers: ieee802154_nrf5: Fix build failure

Fix build failure when CONFIG_NET_L2_OPENTHREAD is not enabled. The
failure looks as follows:

drivers/ieee802154/ieee802154_nrf5.c:187:12:
	warning: 'nrf5_energy_scan_start' defined but not used
	[-Wunused-function]
 static int nrf5_energy_scan_start(struct device *dev,
            ^~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>