phy: msm: ssusb: Power down SSPHY to avoid leakage in probe()

Put SSPHY in power down state explicitly in probe() to avoid
leakage current seen at 1.8V shared LDO between different clients along
with USB on some plaforms, where USB3 is not enabled, even though present.

Change-Id: I577b2d969ee8d4e17e0a3d1f307958a754146738
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>