msm: ipa3: vote turbo when tethering on

Change the modem vote for pm_client as
the highest when usb-tethering is on and vote
back to low power mode when usb-tethering off.

Change-Id: I3e51dc34cea5c97a7f901d9981f908e35b8b0fb2
Signed-off-by: Skylar Chang <chiaweic@codeaurora.org>