USB: serial: mct_u232: fix memory corruption with small endpoint

USB: serial: mct_u232: fix memory corruption with small endpoint

The driver overrides the maximum transfer size for a specific device
which only accepts 16 byte packets for its 32 byte bulk-out endpoint.

Make sure to never increase the maximum transfer size to prevent slab
corruption should a malicious device report a smaller endpoint max
packet size than expected.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>