serial: sh: Improve FIFO empty check on RX

serial: sh: Improve FIFO empty check on RX

If the SCIF is receiving data quickly enough, it may happen that the
SCxSR_RDxF flag is cleared in sh_serial_getc_generic(), while the
FIFO still contains data. If that happens, the serial_getc_check()
reports no data in the FIFO as the flag is no longer set. Add one
more check, if the SCxSR_RDxF is not set, read out the FIFO level
and if there are still characters in the FIFO, permit reading them
out.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>