USB: serial: keyspan_pda: fix receive sanity checks

commit c528fcb116e61afc379a2e0a0f70906b937f1e2c upstream.

Make sure to check for short transfers before parsing the receive buffer
to avoid acting on stale data.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
[bwh: Backported to 3.2:
 - Adjust context
 - Keep the check for !tty in the data case]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --cc drivers/usb/serial/keyspan_pda.c
Simple merge