ns8250: Fix sense of LSR_TEMT FCR check

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: c4b68e7e53bb "ns8250: Check if flush via FCR succeeded"
Differential Revision: https://reviews.freebsd.org/D37185