usb: phy-msm-usb: Return when usb power supply psy is NULL

If usb power supply is not registered, psy would be NULL.
In msm_otg_set_online_status(), add return code when psy is
NULL to prevent NULL pointer dereference.

Change-Id: I03ddfe79c6bddd4eb1a5e5f5bf489c5f5e6baa65
Signed-off-by: Saket Saurabh <ssaurabh@codeaurora.org>