serial: sh-sci: Fix could not remove dev_attr_rx_fifo_timeout

This patch fixes an issue that the sci_remove() could not remove
dev_attr_rx_fifo_timeout because uart_remove_one_port() set
the port->port.type to PORT_UNKNOWN.

Reported-by: Hiromitsu Yamasaki <hiromitsu.yamasaki.ym@renesas.com>
Fixes: 5d23188a ("serial: sh-sci: make RX FIFO parameters tunable via sysfs")
Cc: <stable@vger.kernel.org> # v4.11+
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Ulrich Hecht <uli+renesas@fpond.eu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>