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.

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MFC after: 3 days