iwlwifi: move host command check function into separate file

This patch moves iwl_check_rxon_cmd into iwl-agn-hcmd-check.c
This function compiled out in none debugging or non development
mode and more. We haven't decided which one yet hence preserving the
current 'always compile' state.
More functions will be added to the file namely for checking TX and LQ
commands.

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