TM4C123 UART is always in busy state
It's been Solved. There was a very intricate problem. All registers had perfect values. I re-read the datasheet and came to know that LCRH register should be written after baud rate settings. It doesn't make any difference to the final values of the registers, but somehow it keeps UART in busy state. It wasn't mentioned anywhere in datasheet that what are possible reasons of BUSY flag to be 1. I just re-arranged following lines in my code.

UART0_IBRD_R = 43;                                // IBRD = int(80,000,000 / (16 * 115,200)) = int(43.40277)
UART0_FBRD_R = 26;                                // FBRD = int(0.40277 * 64 + 0.5) = 26                          
UART0_LCRH_R = (UART_LCRH_WLEN_8|UART_LCRH_FEN);  // 8 bit word length (no parity bits, one stop bit, FIFOs)