USB: usb_wwan: fix potential NULL-deref at resume

The interrupt urb was submitted unconditionally at resume, something
which could lead to a NULL-pointer dereference in the urb completion
handler as resume may be called after the port and port data is gone.

Fix this by making sure the interrupt urb is only submitted and active
when the port is open.

Fixes: 383cedc3 ("USB: serial: full autosuspend support for the
option driver")

Cc: <stable@vger.kernel.org>  # v2.6.32: 032129cb
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>