PM / OPP: Update the voltage tolerance when adjusting the OPP

When the voltage is adjusted, the voltage tolerance is not updated.
This can lead to situations where the voltage min value is greater
than the voltage max value. The final result is triggering a BUG()
in the regulator core.
Fix this by updating the voltage tolerance values too.

Signed-off-by: Georgi Djakov <georgi.djakov@linaro.org>