The tty layer uses tsw_busy to poll for busy/idle status of the transmitter hardware during close() and tcdrain(). The ucom layer defines ULSR_TXRDY and ULSR_TSRE bits for the line status register; when both are set, the transmitter is idle. Not all chip drivers maintain those bits in the sc_lsr field, and if the bits never get set the transmitter will always appear busy, causing hangs in tcdrain().
These changes add a new sc_flag bit, UCOM_FLAG_LSRTXIDLE. When this flag is set, ucom_busy() uses the lsr bits to return busy vs. idle state, otherwise it always returns idle (which is effectively what happened before this change because tsw_busy wasn't implemented).
For the uftdi chip driver, these changes stop masking out the tx idle bits when processing the status register (because now they're useful), and it calls ucom_use_lsr_txbits() to indicate the bits are maintained by the driver and can be used by ucom_busy().