USB: Gadget: fix UTF conversion in the usbstring library

This patch (as1234) fixes a bug in the UTF8 -> UTF-16 conversion
routine in the gadget/usbstring library.  In a UTF-8 multi-byte
sequence, all bytes after the first should have their high-order
two bits set to 10, not 11.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>