pinctrl: qcom: fix masking of pinmux functions

The following commit introduced a regression by not properly masking the
calculated value.

Fixes: 47a01ee9 ("pinctrl: qcom: Clear all function selection bits")
Signed-off-by: John Crispin <john@phrozen.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Stephen Boyd <stephen.boyd@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>