serial: sh-sci: Fix fifo stall on SH7760/SH7780/SH7785 SCIF.

There was an off-by-1 in the SCRFDR calculation that caused writes over
128-bytes to hang in the FIFO. Fix it up.

Signed-off-by: Yutaro Ebihara <ebiharaml@si-linux.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>