iwlwifi: move iwl4965_set_rxon_ht into iwlcore

This patch moves iwl4965_set_rxon_ht to iwlcore under name
iwl_set_rxon_ht. It also moves collateral changes
iwl_is_channel_extension and iwl_is_fat_tx_allowed.

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