pinctrl: sh-pfc: r8a7791: Remove bogus marks from vin1_b_data18 group

The vin1_b_data18_mux[] arrays contains pin marks for the 2 LSB bits of
the color components.  The vin1_b_data18_pins[] array rightfully does
not include the corresponding pin numbers, as RGB18 is subset of RGB24,
containing only the 6 MSB bits of each component.

Fixes: 8e32c967 ("pinctrl: sh-pfc: r8a7791: Add VIN pins")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>