iwlwifi: move iwl_[un]init_drv to iwlagn

Since iwlagn is the only user of these functions, move it to this module.
This results in a bit more code moving than just these functions since the
functions only used by them are also moved and we need to export the
symbols previously available to them directly.

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