USB: keyspan: fix potential null pointer dereference

Move control-urb dereference to after NULL-check. There is otherwise a
risk of a possible null pointer dereference.

Was largely found by using a static code analysis program called
cppcheck.

[Johan: modify commit message somewhat ]
[gkh: remove stable tag as it's not a real problem that anyone has ever hit]

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>