serial: sh-sci: Support for variable HSCIF hardware RX timeout

HSCIF has facilities that allow changing the timeout after which an RX
interrupt is triggered even if the FIFO is not filled. This patch allows
changing the default (15 bits of silence) using the existing sysfs
attribute "rx_fifo_timeout".

Signed-off-by: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>