Fix some errors introduced w/ the last commit; fix setting RTS/CTS in the 11n rate scenario.

• I messed up a couple of things in if_athvar.h; so fix that.
• Undo some guesswork done in ar5416Set11nRateScenario() and introduce a flags parameter which lets the caller set a few things. To begin with, this includes whether to do RTS or CTS protection.
• If both RTS and CTS is set, only do RTS. Both RTS and CTS shouldn't be set on a frame.