Like the header says, this patch fixes up the various Transfer- and
Transport-level return codes.  There were a lot of places in the various
subdrivers that were not particularly careful about distinguishing the
two; it would help if the people currently maintaining those drivers could
take a look at my changes to make sure I haven't screwed anything up.

# Converted US_BULK_TRANSFER_xxx to USB_STOR_XFER_xxx, to make it more
# easily distinguishable from USB_STOR_TRANSPORT_xxx.  (Also, in the
# future these codes may apply to control transfers as well as to bulk
# transfers.)
#
# Changed USB_STOR_XFER_FAILED to USB_STOR_XFER_ERROR, since it implies
# a transport error rather than a transport failure.
#
# Added a USB_STOR_XFER_STALLED code, to indicate a transfer that was
# terminated by an endpoint stall.

This patch is in preparation for one in which usb_stor_transfer_partial()
and usb_stor_transfer() are replaced by usb_stor_bulk_transfer_buf() and
usb_stor_bulk_transfer_srb() respectively, with slightly different
argument lists.  Ultimately the subdrivers will be able to use these
routines in place of the slightly specialized versions they have now and
in place of the ones in raw_bulk.c.