msm: ipa: register with PM for USB only once

Register with IPA PM framework only once for all USB tethering
protocols.

Change-Id: I65fa965ac493762c1ef76db8a8344b8e1e419366
CR-Fixed: 2158711
Acked-by: Ady Abraham <adya@qti.qualcomm.com>
Signed-off-by: Skylar Chang <chiaweic@codeaurora.org>