hw/char/sh_serial: Return correct number of empty RX FIFO elements

hw/char/sh_serial: Return correct number of empty RX FIFO elements

In the IOCanReadHandler sh_serial_can_receive(), if the Serial
Control Register 'Receive Enable' bit is set (bit 4), then we
return a size of (1 << 4) which happens to be equal to 16, so
effectively SH_RX_FIFO_LENGTH.

The IOReadHandler, sh_serial_receive1() takes care to receive
multiple chars, but if the FIFO is partly filled, we only process
the number of free slots in the FIFO, discarding the other chars!

Fix by returning how many elements the FIFO can queue in the
IOCanReadHandler, so we don't have to process more than that in
the IOReadHandler, thus not discarding anything.

Remove the now unnecessary check on 's->rx_cnt < SH_RX_FIFO_LENGTH'
in IOReadHandler, reducing the block indentation.

Fixes: 63242a007a1 ("SH4: Serial controller improvement")
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Luc Michel <luc.michel@amd.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20250220092903.3726-10-philmd@linaro.org>