When TXBRK is set in LCRH and UART is idle, TXD pin is driven low and kept low
until SW clears TXBRK bit.
When TXBRK is set when transmission is active, ongoing data packet will be sent
out and thenTXD pin is driven low and kept low until TXBRK is cleared by SW.
If TXFIFO contains data, transmission will resume once TXBRK is cleared by SW.
SW recommendation is to keep TXBRK set for two times the length of data packet
for proper break operation.