mac80211.h: fix kernel-doc excesses

Fix mac80211.h kernel-doc: it had some extra parameters that were
no longer valid and incorrect format for a return value in 2 places.

Warning(lin2628-rc2//include/net/mac80211.h:1487): Excess function parameter or struct member 'control' description in 'ieee80211_beacon_get'
Warning(lin2628-rc2//include/net/mac80211.h:1596): Excess function parameter or struct member 'control' description in 'ieee80211_get_buffered_bc'
Warning(lin2628-rc2//include/net/mac80211.h:1632): Excess function parameter or struct member 'rc4key' description in 'ieee80211_get_tkip_key'
Warning(lin2628-rc2//include/net/mac80211.h:1735): Excess function parameter or struct member 'return' description in 'ieee80211_start_tx_ba_session'
Warning(lin2628-rc2//include/net/mac80211.h:1775): Excess function parameter or struct member 'return' description in 'ieee80211_stop_tx_ba_session'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>