When flushing the UART, we need to drain manually if LSR_TEMT is
*not* asserted, aka. if the transmit FIFO is not empty.
Reported by: void <void@f-m.fm>
Fixes: c4b68e7e53 "ns8250: Check if flush via FCR succeeded"
Differential Revision: https://reviews.freebsd.org/D37185