net: dpaa2-eth: create and export the dpaa2_eth_receive_skb() function

Carve out code from the dpaa2_eth_rx() function in order to create and
export the dpaa2_eth_receive_skb() function. Do this in order to reuse
this code also from the XSK path which will be introduced in a later
patch.

Signed-off-by: Robert-Ionut Alexa <robert-ionut.alexa@nxp.com>
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>