USB: msm_otg: Return correct error code from probe

VBUS regulator_get may fail with EPROBE_DEFER. In this case
driver should return same error code from probe so that kernel
retries calling driver's probe routine later. Otherwise, driver
probe would never be called even if vbus regulator gets added
later.

Change-Id: I3b05a04417c4ba728aa698ed108b14c8c86849f9
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>