usb: cdc-wdm: Constify static struct wwan_port_ops

usb: cdc-wdm: Constify static struct wwan_port_ops

The only usage of wdm_wwan_port_ops is to pass its address to
wwan_create_port() which takes a pointer to const wwan_port_ops as
argument. Make it const to allow the compiler to put it in read-only
memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Acked-by: Oliver Neukum <oneukum@suse.com>
Link: https://lore.kernel.org/r/20210929194547.46954-3-rikard.falkeborn@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>