USB: keyspan: add a sanity test on "len"

"len" comes from the USB transfer and it's probably correct.  The thing
is that we already have similar checks like:

        if (data[i] >= serial->num_ports) {

So adding a sanity test here matches the rest of the code and is a good
idea.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>