serial: sh-sci: Use DEVICE_ATTR_RW() for rx_fifo_trigger

While commit b6b996b6 ("treewide: Use DEVICE_ATTR_RW") converted
the rx_fifo_timeout attribute, it forgot to convert rx_fifo_trigger due
to a slightly different function naming.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Ulrich Hecht <uli+renesas@fpond.eu>
Link: https://lore.kernel.org/r/20190731124555.14349-1-geert+renesas@glider.be


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>