msm: rpm-regulator-smd: limit current to RPM defined maximum

Modify the rpm_vreg_get_optimum_mode() function so that it
restricts the maximum current that can be sent for the 'ma' KVP
to the limit defined by the RPM.

Change-Id: I744e656c75a5e78a81df9c283a8f14a977c4c6ee
Signed-off-by: David Collins <collinsd@codeaurora.org>