pinctrl: qcom: Clear all function selection bits

The function selection bitfield is not always 3 bits wide.
Sometimes it is 4 bits wide. Let's use the npins struct member to
determine how many bits wide the function selection bitfield is
so we clear the correct amount of bits in the register while
remuxing the pins.

Cc: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Stephen Boyd <stephen.boyd@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>