Don't memset the TXRX buffer when radio is set to receive mode

[dberlin]

The 1262 doesn't do this memset, but the 1272/1276 do. This means, in an interrupt, if the radio is put back in RX mode before the payload is extracted and handed to a callback, it is zero'd. This causes bugs like https://github.com/zephyrproject-rtos/zephyr/pull/48765 They would prefer it be fixed here, as it is just a waste anyway - we don't do this in 1262, and don't need to do it here.