iwlwifi: update lowest API version support for 6x00 & 6x50 series

For 6x00 and 6x50 series devices, APIv4 is the lowest firmware version
driver can support. This is also the lowest API version available to the
public so there is no need for backward compatibility support for the
earlier API versions.

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