ath9k: use ieee80211_conf on ath9k_hw_iscal_supported()

ath9k_hw_iscal_supported() just needs to be aware of your band
and if HT20 is being used so lets abandon our internal channel,
HT appended values and internal mode values and use ieee80211_conf
which already carries this information. This works as calibration
is being done for the currently configured channel.

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