usb: cdc-wdm: Fix check for WWAN

CONFIG_WWAN_CORE was with CONFIG_WWAN in commit 89212e16 ("net: wwan:
Fix WWAN config symbols"), but did not update all users of it. Change it
back to use CONFIG_WWAN instead.

Fixes: 89212e16

 ("net: wwan: Fix WWAN config symbols")
Cc: <stable@vger.kernel.org>
Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Acked-by: Oliver Neukum <oneukum@suse.com>
Link: https://lore.kernel.org/r/20210929194547.46954-2-rikard.falkeborn@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>