TI CC2538 radio turns off after connecting UART
I would suggest that you test the setup and hardware first with foundation firmware available for cc2538. In the foundation firmware in the UART configuration just intialise the radio with continuous transmit mode(FRMCTRL1 register o 0x43) and send some dummy packet on UART. The other way to check the setup is use the ZNP workspace from the Smart Light zstack download image from ti.com. As soon as you will burn the ZNP image in CC2538 and power up you can see the data on UART(by default 115200 baud) of CC2538 which will confirm the correct connections and hardware setup.