USB: separate out endpoint queue management and DMA mapping routines

This patch (as953) separates out three key portions from
usb_hcd_submit_urb(), usb_hcd_unlink_urb(), and usb_hcd_giveback_urb()
and puts them in separate functions of their own.  In the next patch,
these functions will be called directly by host controller drivers
while holding their private spinlocks, which will remove the
possibility of some unpleasant races.

The code responsible for mapping and unmapping DMA buffers is also
placed into a couple of separate subroutines, for the sake of
cleanliness and consistency.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>