drivers: ieee802154: nrf5: support raw mode

When CONFIG_IEEE802154_RAW_MODE is set there is no network interface
that could provide pointer to the device the interface is running on top
of. The current implementation of nRF5 ieee802154 driver implicitly
assumes that such an interface is always present, which leads to crashes
when raw mode is enabled.

This commit adds support for IEEE802154_RAW_MODE in nRF5 ieee802154
driver by latching pointer to the ieee802154 device on initialization if
needed so that it doesn't have to be retrieved using the network
interface in run-time.

Signed-off-by: Jędrzej Ciupis <jedrzej.ciupis@nordicsemi.no>