ethdev: introduce generic dummy packet burst function

ethdev: introduce generic dummy packet burst function

Multiple PMDs have dummy/noop Rx/Tx packet burst functions.

These dummy functions are very simple, introduce a common function in
the ethdev and update drivers to use it instead of each driver having
its own functions.

Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Morten Brørup <mb@smartsharesystems.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>