serial: sh-sci: Drop useless check for zero sampling_rate

sci_port.sampling_rate is always non-zero, except for HSCIF, which uses
sci_baud_calc_hscif() instead of sci_scbrr_calc().

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>