iavf: Implement checking DD desc field

Rx timestamping introduced in PF driver caused the need of refactoring
the VF driver mechanism to check packet fields.

The function to check errors in descriptor has been removed and from
now only previously set struct fields are being checked. The field DD
(descriptor done) needs to be checked at the very beginning, before
extracting other fields.

Reviewed-by: Rahul Rameshbabu <rrameshbabu@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Tested-by: Rafal Romanowski <rafal.romanowski@intel.com>
Signed-off-by: Mateusz Polchlopek <mateusz.polchlopek@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>