net: lan966x: Split function lan966x_fdma_rx_get_frame

The function lan966x_fdma_rx_get_frame was unmapping the frame from
device and check also if the frame was received on a valid port. And
only after that it tried to generate the skb.
Move this check in a different function, in preparation for xdp
support. Such that xdp to be added here and the
lan966x_fdma_rx_get_frame to be used only when giving the skb to upper
layers.

Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>