tty: Fix leak in ti-usb

If the ti-usb adapter returns an zero data length frame (which happens)
then we leak a kref.  Found by Christoph Mair <christoph.mair@gmail.com>
who proposed a patch.  The patch here is different as Christoph's patch
didn't work for the case where tty = NULL and data arrived but Christoph
did all the hard work chasing it down.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>