nfp: drop rx_ring param from buffer allocation

We will soon allocate RX buffers for caching on XDP TX rings.
The rx_ring parameter passed to nfp_net_rx_alloc_one() is not
actually used, remove it.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>