ath9k: use hw->conf on ath_setcurmode()

We don't need to use our own mode for setting the
the routine tries to do, in fact lets remove ath_chan2mode() now as
we can simply use the currently set band and the HT configuration
provided by mac80211 through the ieee80211_conf. This works on
changing channels as well as the internal ath9k_channel we use is
based on the ieee80211_channel in the config.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>