regulator: fix incorrectly cached voltage on error

In regulator_set_voltage, if _regulator_do_set_voltage returns
an error, the new voltage will still be cached.  If
regulator_set_voltage is called again with the same constraints,
it will return success, but the voltage will still be at the
old voltage.

Keep the old voltage cached if _regulator_do_set_voltage returns
and error.

Signed-off-by: Colin Cross <ccross@android.com>
Change-Id: I7c9cb3e508be92a3ec807e88f093a8e185e56b3c