serial: sh-sci: Fix too early port disabling.

Currently serial ports on SH CPUs get disabled too early, because the
sci_tx_empty() routine claims to not be able to detect whether the
transmission has been completed and just always returns TIOCSER_TEMT. This
results in corrupt output of last characters if the port is not open for
reading at the same time. It is however possible to detect whether
transmission has been completed. Use the TEND bit of the status register
for this.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>