USB: fix LANGID=0 regression

commit b7af0bb2 ("USB: allow malformed LANGID descriptors") broke support
for devices without string descriptor support.

Reporting string descriptors is optional to USB devices, and a device
lets us know it can't deal with strings by responding to the LANGID
request with a STALL token.

The kernel handled that correctly before b7af0bb2

 came in, but failed
hard if the LANGID was reported but broken. More than that, if a device
was not able to provide string descriptors, the LANGID was retrieved
over and over again at each string read request.

This patch changes the behaviour so that

 a) the LANGID is only queried once
 b) devices which can't handle string requests are not asked again
 c) devices with malformed LANGID values have a sane fallback to 0x0409

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>