pinctrl: SPEAr: Don't update all non muxreg bits on pinctrl_disable

Not all bits of a register are used for pinctrl in SPEAr. So only update bits
relevant to pinctrl using muxreg->mask.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>