drivers: ieee802154_nrf5: fix condition for mac keys and frame counter

For multicore devices like nRF53, shim layer part of setting mac keys
and frame counters is called on application core where
CONFIG_NRF_802154_ENCRYPTION is disabled (the define concerns radio
driver for net core). This commit replaces
CONFIG_NRF_802154_ENCRYPTION with CONFIG_IEEE802154_2015.

Signed-off-by: Lukasz Maciejonczyk <lukasz.maciejonczyk@nordicsemi.no>