drivers: ieee802154: nrf5: drop packets that are too long

There was an observed situation where the assert checking frame length
is below 128 bytes was hit. While the nrf-802154 checks that the frame
does not exceed that size, there might exists code paths that allow such
packets to pass through.

The change removes the assert and drops the packet instead.
An error is also printed to allow for further debugging.

Signed-off-by: Rafał Kuźnia <rafal.kuznia@nordicsemi.no>