virt_wifi: remove set but not used variable 'w_priv'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/wireless/virt_wifi.c: In function 'virt_wifi_net_device_stop':
drivers/net/wireless/virt_wifi.c:432:31: warning:
 variable 'w_priv' set but not used [-Wunused-but-set-variable]

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>