serial: sh-sci: Stop RX FIFO timer during port shutdown

The RX FIFO timer may be armed when the port is shut down, hence the
timer function may still be called afterwards.

Fix this race condition by deleting the timer during port shutdown.

Fixes: 03940376 ("serial: sh-sci: SCIFA/B RX FIFO software timeout")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>