Gcc report build error as following when CONFIG_WWAN_CORE is not set:

x86_64-linux-gnu-ld: drivers/usb/class/cdc-wdm.o: in function `wdm_disconnect':
cdc-wdm.c:(.text+0xb2a): undefined reference to `wwan_remove_port'
x86_64-linux-gnu-ld: drivers/usb/class/cdc-wdm.o: in function `wdm_in_callback':
cdc-wdm.c:(.text+0xf23): undefined reference to `wwan_port_rx'
x86_64-linux-gnu-ld: drivers/usb/class/cdc-wdm.o: in function `wdm_wwan_port_stop':
cdc-wdm.c:(.text+0x127d): undefined reference to `wwan_port_get_drvdata'
x86_64-linux-gnu-ld: drivers/usb/class/cdc-wdm.o: in function `wdm_wwan_port_tx':
cdc-wdm.c:(.text+0x12d9): undefined reference to `wwan_port_get_drvdata'
x86_64-linux-gnu-ld: cdc-wdm.c:(.text+0x13c1): undefined reference to `wwan_port_txoff'
x86_64-linux-gnu-ld: drivers/usb/class/cdc-wdm.o: in function `wdm_wwan_port_start':
cdc-wdm.c:(.text+0x13e0): undefined reference to `wwan_port_get_drvdata'
x86_64-linux-gnu-ld: cdc-wdm.c:(.text+0x1431): undefined reference to `wwan_port_txon'
x86_64-linux-gnu-ld: drivers/usb/class/cdc-wdm.o: in function `wdm_wwan_port_tx_complete':
cdc-wdm.c:(.text+0x14a4): undefined reference to `wwan_port_txon'
x86_64-linux-gnu-ld: drivers/usb/class/cdc-wdm.o: in function `wdm_create.cold':
cdc-wdm.c:(.text.unlikely+0x209): undefined reference to `wwan_create_port'

Using CONFIG_WWAN_CORE instead of CONFIG_WWAN to avoid build error.

Fixes: cac6fb01 ("usb: class: cdc-wdm: WWAN framework integration")
Reported-by: Hulk Robot <hulkci@huawei.com>
Reviewed-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Link: https://lore.kernel.org/r/20210521021010.2490930-1-weiyongjun1@huawei.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>