LinuxKPI: 802.11: fix for_each_sta_active_link()

Likely a c&p error from for_each_vif_active_link() to
for_each_sta_active_link().
We are checking the nitems on the vif instead of the sta in this macro.
Function wise there is no difference as the arrays are the same size
but for correctness fix this.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 69b6c4a6ec6654978628ccd48edce46f00ac3e96)