xhci: Avoid infinite loop when sg urb requires too many trbs

Currently prepare_ring() returns -ENOMEM if the urb won't fit into a
single ring segment.  usb_sg_wait() treats this error as a temporary
condition and will keep retrying until something else goes wrong.

The number of retries should be limited in usb_sg_wait(), but also
prepare_ring() should not return an error code that suggests it might
be worth retrying.  Change it to -EINVAL.

Reported-by:  <jidanni@jidanni.org>
References: http://bugs.debian.org/733907


Fixes: 35773dac ('usb: xhci: Link TRB must not occur within a USB payload burst')
Cc: stable <stable@vger.kernel.org> # 3.12
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>