usb: check usb_hub_to_struct_hub() return value

usb_hub_to_struct_hub() can return NULL in some unlikely cases.
Add checks where appropriate, or pass the hub pointer as an additional
argument if it's known to be valid.

The places it makes sense to check usb_hub_to_struct_hub()
are picked based on feedback from Alan Stern.

Change-Id: I10103d23876934c7783e241f5a4a07587860822d
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Git-commit: 41341261aa1707b49f937ba2c20d1a0daa5afac3
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


[mrana@codeaurora.org: resolve trivial merge conflicts]
CRs-Fixed: 562361
Signed-off-by: Mayank Rana <mrana@codeaurora.org>