Staging: wlan-ng: Fix sparse non-static symbol warning

Fix sparse non-static symbol warning in wlan-ng driver.

Signed-off-by: Tugce Sirin <ztugcesirin@gmail.com>
Acked-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>