drivers: ieee802154: rf2xx: Fix OT ACK handling

At OpenThread stack on ieee802154_radio_handle_ack method the
net_pkt_read fail because net_pkt_cursor wasn't proper initialized.
This ensures that net_pkt_cursor is initialized for any ack frame.

Another hidden problem fixed was the frags property on net_buf. Now
it is defined as NULL to ensures that no fragments are available.

The ack frame should be returned to OT only if requested. In this
case, the IEEE 802.15.4 Frame Control field from TX frag is now
verified for an ACK request and if OT requires ACK response it will
be proper handled and dispatched.

Fixes #23595

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