USB: usb_wwan: fix potential NULL-deref at resume

commit 9096f1fbba916c2e052651e9de82fcfb98d4bea7 upstream.

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")

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