usb: Change usb_hcd->bandwidth_mutex to a pointer.

Change the bandwith_mutex in struct usb_hcd to a pointer.  This will allow
the pointer to be shared across usb_hcds for the upcoming work to split
the xHCI driver roothub into a USB 2.0/1.1 and a USB 3.0 bus.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>