ath5k: honor the RTS/CTS bits

The ath5k driver didn't use set_rts_threshold or use_cts_prot, and also
didn't check the IEEE80211_TX_RC_USE_{RTS_CTS,CTS_PROTECT} RC flags.
Tell the hardware about these so RTS/CTS will work, and so the device
will work better in mixed b/g environments.

Changes-licensed-under: 3-Clause-BSD

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>