msm: 9615: regulators: Move USB VDDCX to corner voting

This change will allow USB & HSIC drivers to use corner
voting for VDDCX, instead of specific voltage level voting.

Change-Id: I8bc983eb4de4b51c0ec0cbc9d19a6445fd13beb5
CRs-Fixed: 354458
Signed-off-by: Amit Blay <ablay@codeaurora.org>
Signed-off-by: David Collins <collinsd@codeaurora.org>