There's currently no way to re-enable a mpp block once you've entered a state
that disables the state, this patch makes it possible to leave the
bias-high-impedance state.

Also read the enable state from the hardware on probe.

With this in place the is_enabled variable is accurately tracking the state of
the hardware and we can use that for the debug output as well.
Signed-off-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>