msm: rpm-regulator-smd: Remove list_voltage() callback function

Remove the list_voltage() callback function from the regulator
ops struct which is passed into regulator_register() in the
rpm-regulator-smd driver.  Since this driver is unaware of which
regulator voltages correspond to physically available set points,
it is not possible to have a meaningful list_voltage() callback
function.

Continue specifying a non-zero n_voltages value for
rpm-regulator-smd devices of type other than voltage switch so
that consumers can detect if regulator_set_voltage() is supported
by checking if regulator_count_voltages() returns greater than 0.

Change-Id: I24356785647e528b9523503dd25b43ac32abbc5f
Signed-off-by: David Collins <collinsd@codeaurora.org>