iwlwifi: align set channel with mac80211

This patch makes use of mac80211 defaults for channel and band.
Older code that uses G band channel 6 was removed. This patch also
changes iwl_set_rxon_channel signature.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>