iwlwifi: wimax co-exist code clean up

wifi/wimax co-exist command is part of _agn device configuration
sequence; move it to iwl-agn-ucode.c which is more appropriate place for the
function.

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>