drivers: ieee802154: rf2xx: Add missing handle_ack call

The rf2xx driver is doing automatic retransmissions in hardware based
on whether ACKs are received or not. Currently the driver is not
invoking ieee802154_radio_handle_ack() as other drivers are doing and
required by OpenThread since 4fe1da9f. This add rf2xx_handle_ack method
to ensures required ACK processing when driver performs TX.

Fixes: #21763

Signed-off-by: Gerson Fernando Budke <nandojve@gmail.com>