regulator: qpnp-regulator: maintain current voltage range whenever possible

Voltage spikes may occur if the voltage range of a regulator is
changed while the regulator is enabled.  Therefore, modify the
voltage setting function so that the current voltage range is
used whenever possible.

Change-Id: I13dda99c751acfe083f2bcaacf09348d317fc0e7
Signed-off-by: David Collins <collinsd@codeaurora.org>