usb: xhci: Fix a potential NULL pointer dereference

This patch fixes a potential NULL pointer dereference arising on
non-present/non-initialized xHCI controllers and adds some error
handling to xHCI code

Signed-off-by: Sergey Temerkhanov <s.temerkhanov@gmail.com>
Signed-off-by: Radha Mohan Chintakuntla <rchintakuntla@cavium.com>