USB: HCD: Add usb_hcd prefix to exported functions

The convention is to prefix symbols exported from the USB HCD core with
"usb_hcd".  This change makes unmap_urb_setup_for_dma() and
unmap_urb_for_dma() consistent with that.

Signed-off-by: Robert Morell <rmorell@nvidia.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>