USB: serial: keyspan_pda: fix receive sanity checks

USB: serial: keyspan_pda: fix receive sanity checks

BugLink: http://bugs.launchpad.net/bugs/1691418
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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>