ethdev: allow to force Rx scatter mode

ethdev: allow to force Rx scatter mode

We might want to be sure the scatter packets reception handler is selected in a
pmd. This makes it possible to then change mtu later, without the need of
restarting a port.
It is then the pmd duty to tell it enabled the scatter reception handler by
setting dev->data->scattered_rx to 1.

Signed-off-by: David Marchand <david.marchand@6wind.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>