regulator: qpnp-regulator: Assign selector value in set_voltage() callbacks

Assign a value to the selector pointer that is passed into
set_voltage() callback functions in the qpnp-regulator driver.
The value assigned to the selector pointer should correctly map
to the regulator voltage through the list_voltage() callback
function.

Change-Id: I7d72feb77e6122982a46c626741450108742fb8b
Signed-off-by: David Collins <collinsd@codeaurora.org>