mac80211: remove the repeated declaration

Function 'ieee80211_sta_set_rx_nss' is declared twice, so remove the
repeated declaration.

Signed-off-by: Shaokun Zhang <zhangshaokun@hisilicon.com>
Link: https://lore.kernel.org/r/1622196424-62403-1-git-send-email-zhangshaokun@hisilicon.com


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