iwlwifi: get_hw_cmd_size

This patch introduces a new handler get_hw_cmd_size in hcmd_utils,
which should adjust the size of the command sent to the microcode
according to the current nic.
It also changes the RXON flow to make usage of this new handler.

The patch also adds iwl_rxon_cmd structure which is supperset for
5000 HW and 4965.

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