usb: dwc3-msm: Get otg_xceiv directly from dwc3_otg

Obtain a pointer to the usb_otg structure directly via the core
dwc3 structure rather than using usb_get_phy() to retrieve it.
By doing so this avoids the potential of getting an incorrect
PHY registered by another driver (if multiple USB instances are
present), and clumsily having to identify if it's the correct one.
Also remove the dwc3_otg_register_phys() function as the OTG PHY
now no longer needs to be registered.

Change-Id: Id8c9dd265893b3c798594d980af2835ab1e40b02
Signed-off-by: Jack Pham <jackp@codeaurora.org>