regulator: core: Allow for regulators that can't be read at bootup

Regulators attached via RPMh on Qualcomm sdm845 apparently are
write-only.  Specifically you can send a request for a certain voltage
but you can't read back to see what voltage you've requested.  What
this means is that at bootup we have absolutely no idea what voltage
we could be at.

As discussed in the patches to try to support the RPMh regulators [1],
the fact that regulators are write-only means that its driver's
get_voltage_sel() should return an error code if it's called before
any calls to set_voltage_sel().  This causes problems in
machine_constraints_voltage() when trying to apply the constraints.

A proposed fix was to come up with an error code that could be
returned by get_voltage_sel() which would cause the regulator
framework to simply try setting the voltage with the current
constraints.

In this patch I propose the error code -ENOTRECOVERABLE.  In errno.h
this error is described as "State not recoverable".  Though the error
code was originally intended "for robust mutexes", the description of
the error code seems to apply here because we can't read the state of
the regulator.  Also note that the only existing user of this error
code in the regulator framework is tps65090-regulator.c which returns
this error code from the enable() call (not get_voltage() or
get_voltage_sel()), so there should be no existing regulators that
might accidentally get the new behavior.  (Side note is that tps65090
seems to interpret this error code to mean an error that you can't
recover from rather than some data that can't be recovered).

[1] https://patchwork.kernel.org/patch/10340897/



Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>