iwlwifi: move iwl_is_ht40_tx_allowed out of iwl-core.c

This is really something determined by
station parameters, so move it to
iwl-agn-sta.c.

Signed-off-by: Meenakshi Venkataraman <meenakshi.venkataraman@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>