Staging: wlan-ng: Fix smatch warning potential null reference

This patch fixes potential null dereference 'wlandev' in wlan-ng driver.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Tugce Sirin <ztugcesirin@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>