packet: remove handling of tx_ring from prb_shutdown_retire_blk_timer()

Follow e8e85cc5 ("packet: remove handling of tx_ring") and remove
the tx_ring parameter from prb_shutdown_retire_blk_timer() as it is only
called with tx_ring = 0.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>