iwl3945: Getting rid of the *39_rxon iwl_priv fields

The iwl_rxon_cmd is really just a iwl3945_rxon_cmd structure extension.
So, we can use the *_rxon fields from iwl_priv instead of the 3945 specific
ones (*39_rxon). We have to then be careful when submitting REPLY_RXON host
commands, since the command length as to be set according to the HW. As
another precaution the reserved4 and reserved5 fields are cleared before being
sent to the 3945.

With the *39_rxon removal, a lot of duplicated code can be removed from the
3945 code base.

Signed-off-by: Samuel Ortiz <samuel.ortiz@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>