net/iavf: use write combining store for tail updates

net/iavf: use write combining store for tail updates

Performance improvement: use a write combining store
instead of a regular mmio write to update queue tail
registers.

Signed-off-by: Gordon Noonan <gordon.noonan@intel.com>
Signed-off-by: Radu Nicolau <radu.nicolau@intel.com>
Acked-by: Qi Zhang <qi.z.zhang@intel.com>