pinmux: forbid mux_usecount to be set at UINT_MAX

If pin_free is called on a pin already freed, mux_usecount is set to
UINT_MAX which is really a bad idea.

This will issue a warning, so that we can correct the code responsible
for the double free.

Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>