pinctrl: single: fix pcs_disable with bits_per_mux

pcs_enable() uses vals->mask instead of pcs->fmask when bits_per_mux is
enabled. However, pcs_disable() always uses pcs->fmask.

Fix pcs_disable() to use vals->mask with bits_per_mux.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>