USB: avoid using urb->pipe in usbcore

This patch (as946) eliminates many of the uses of urb->pipe in
usbcore.  Unfortunately there will have to be a significant API
change, affecting all USB drivers, before we can remove it entirely.
This patch contents itself with changing only the interface to
usb_buffer_map_sg() and friends: The pipe argument is replaced with a
direction flag.  That can be done easily because those routines get
used in only one place.

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