drivers: ieee802154: nrf: cache radio channel

Cache configured radio channel and apply them only when a relevant
radio task is requested.

This allows to configure the channel in the transmit metadata, thus
avoiding unneeded `nrf_802154` API calls in some scenarios.

Signed-off-by: Eduardo Montoya <eduardo.montoya@nordicsemi.no>
Signed-off-by: Jędrzej Ciupis <jedrzej.ciupis@nordicsemi.no>