net/packet: remove dead code and unneeded variable from prb_setup_retire_blk_timer()

We test for 'tx_ring' being != zero and BUG() if that's the case. So after
that check there is no way that 'tx_ring' could be anything _but_ zero, so
testing it again is just dead code. Once that dead code is removed, the
'pkc' local variable becomes entirely redundant, so remove that as well.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: David S. Miller <davem@davemloft.net>