USB: mct_u232: add sanity checking in probe

USB: mct_u232: add sanity checking in probe

commit 4e9a0b05257f29cf4b75f3209243ed71614d062e upstream.

An attack using the lack of sanity checking in probe is known. This
patch checks for the existence of a second port.

CVE-2016-3136

Signed-off-by: Oliver Neukum <ONeukum@suse.com>
[johan: add error message ]
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[bwh: Backported to 3.2: put the check in mct_u232_startup(), which already
 has a 'serial' variable]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>