ice: refactor ice_ddp to make functions static

As following methods are not used outside of ice_ddp,
they can be made static:
ice_verify_pgk
ice_pkg_val_buf
ice_aq_download_pkg
ice_aq_update_pkg
ice_find_seg_in_pkg

Signed-off-by: Jan Sokolowski <jan.sokolowski@intel.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>