Revert "serial: core: only stop transmit when HW fifo is empty"

commit c5603e2a upstream.

This reverts commit 7bfb915a.

This commit broke pxa and omap-serial, because it inhibited them from
calling stop_tx() if their TX FIFOs weren't completely empty. This
resulted in these two drivers hanging during transmits because the TX
interrupt would stay enabled, and a new TX interrupt would never fire.

Cc: stable@vger.kernel.org
Fixes: 7bfb915a

 ("serial: core: only stop transmit when HW fifo is empty")
Signed-off-by: Doug Brown <doug@schmorgal.com>
Link: https://lore.kernel.org/r/20240606195632.173255-2-doug@schmorgal.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>