tsnep: Move skb receive action to separate function

The function tsnep_rx_poll() is already pretty long and the skb receive
action can be reused for XSK zero-copy support. Move page based skb
receive to separate function.

Signed-off-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>