mac80211: trivial spelling fixes

Fix spelling and readability of a few lines of kernel doc:

    s/issueing/issuing/g
    s/approriate/appropriate/g
    s/supported by simply/supported simply by/
    s/IEEE80211_HW_BEACON_FILTERING/IEEE80211_HW_BEACON_FILTER/g

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>