Revert "regulator: rpm-smd-regulator: don't wait for ACK when lowering power state"

This reverts commit d9556f3b
("regulator: rpm-smd-regulator: don't wait for ACK when lowering
power state") which conditionally avoids calling
msm_rpm_wait_for_ack() after msm_rpm_send_request().  The lack
of msm_rpm_wait_for_ack() calls can result in the device not
being able to enter suspend due to dependencies within the
rpm-smd driver.

Change-Id: Ibd3f5c1877090e9101d42f07c473348a0e5c1a38
Signed-off-by: David Collins <collinsd@codeaurora.org>