This commit sets the correct value to the MCU_CTRL bit for the
cc2538_rf. This variable is used to detect when the radio is doing
transmission with CCA. Since this was not set to 1 when sending without
CCA, the `confirm_transmit` function returned -EAGAIN after the TX_DONE
event (which violates the HAL API).
This error was discovered using some Radio HAL riotctrl scripts by @LarsKowoll.
3c56cdf0c0