mux: add missing mux_state_get

And implement devm_mux_state_get in terms of the new function.

Now we have both mux_state_get and mux_state_put as convenient functions
ready to be exported should someone ever need unmanaged interfaces.

Tested-by: Aswath Govindraju <a-govindraju@ti.com>
Signed-off-by: Peter Rosin <peda@axentia.se>
Link: https://lore.kernel.org/r/6f8cfdfd-9fa6-40d1-09b3-0c9fc50835ac@axentia.se


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>