serial: sh: Handle HSCIF RX FIFO overflow

serial: sh: Handle HSCIF RX FIFO overflow

The HSCIF RX FIFO may overflow when data are streaming from remote end
into the HSCIF while U-Boot is still starting up. In that case, HSFSR
bit RDF is set, but HSFDR field R is zero. This confuses .tstc callback
into considering RX FIFO to be empty, which leads to .getc to be never
invoked, even when user attempts to pass more input onto the command
line.

Fix this by considering the RDF flag in serial_rx_fifo_level(), which
is called from .tstc in case of no errors. If RDF flag is set, trigger
the .getc callback and let it clear the RX FIFO.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>