regulator: stub: Initialize voltage to min_uV constraint

The regulator_set_optimum API expects a non-zero voltage
returned from the get_voltage regulator_op. The current
default voltage is initialized to zero, and so this agreement
is broken.

Change-Id: I0c225604803cd633348070d2fe02a5cd8c4a45e1
Signed-off-by: Michael Bohan <mbohan@codeaurora.org>