i40e: fix ieee1588 timestamping with next ABI

i40e: fix ieee1588 timestamping with next ABI

Fixes issue where ieee15888 timestamping doesn't work for the i40e
pmd when RTE_ABI_NEXT is enabled.

Also refactors repeated ieee15888 flag checking and setting
code into a function.

Test report: http://dpdk.org/ml/archives/dev/2015-August/022496.html

Reported-by: Huilong Xu <huilongx.xu@intel.com>
Signed-off-by: John McNamara <john.mcnamara@intel.com>
Acked-by: Sergio Gonzalez Monroy <sergio.gonzalez.monroy@intel.com>
Acked-by: Wenzhuo Lu <wenzhuo.lu@intel.com>
Tested-by: Huilong Xu <huilongx.xu@intel.com>