usbcore: check usb_hub_to_struct_hub() return value

usb_hub_to_struct_hub() can return NULL in some unlikely cases.
Add checks where appropriate and handle error case properly.

CRs-Fixed: 562361
Change-Id: I09a11a08a8fa71ca6ecdb3a42ba0e6c2503ee0c0
Signed-off-by: Mayank Rana <mrana@codeaurora.org>