mux: add mux_control_get_optional() API

Sometimes drivers only use muxes under certain scenarios. For
example, the chipidea usb controller may be connected to a usb
switch on some platforms, and connected directly to a usb port on
others. The driver won't know one way or the other though, so add
a mux_control_get_optional() API that allows the driver to
differentiate errors getting the mux from there not being a mux
for the driver to use at all.

Signed-off-by: Stephen Boyd <stephen.boyd@linaro.org>
[minor adjustments to logic, comments and made a function static]
Signed-off-by: Peter Rosin <peda@axentia.se>