Add tsw_busy support to usb_serial (ucom).

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().

Differential Revision: https://reviews.freebsd.org/D9183