net/i40e: use relaxed and remove duplicate barrier

net/i40e: use relaxed and remove duplicate barrier

To guarantee the orderings of successive stores to CIO and MMIO memory,
a lighter weight rte_io_wmb [1] can be used instead of rte_wmb, and since
the I40E_PCI_REG_WRITE API already has an inclusive rte_io_wmb, this
explicit call can be even saved.

[1] http://git.dpdk.org/dpdk/tree/lib/librte_eal/common/include/generic/
rte_atomic.h#n98

Signed-off-by: Gavin Hu <gavin.hu@arm.com>
Acked-by: Qi Zhang <qi.z.zhang@intel.com>