pinctrl: msm: drop negativity check on unsigned value

[linux-3.16-rc5/drivers/pinctrl/pinctrl-msm.c:145]: (style) Checking if
unsigned variable 'mux_bit' is less than zero.

    if (WARN_ON(g->mux_bit < 0))
        return -EINVAL;

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=80491


Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Andrey Utkin <andrey.krieger.utkin@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>