ath: Make ath_opmode_to_string understand OCB mode

Make ath_opmode_to_string return "OCB" for NL80211_IFTYPE_OCB. Currently
it will return "UNKNOWN".

Signed-off-by: Bertold Van den Bergh <bertold.vandenbergh@esat.kuleuven.be>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>