plip: use netif_rx_ni() for packet receive

netif_rx is meant to be called from interrupts because it doesn't wake
up ksoftirqd.  For calling from outside interrupts, netif_rx_ni exists.

This fixes plip to use netif_rx_ni.  It fixes the infamous error "NOHZ:
local_softirq_panding 08" that happens on some machines with NOHZ and
plip --- it is caused by the fact that softirq is pending and ksoftirqd
is sleeping.

Signed-off-by: Mikulas Patocka <mikulas@artax.karlin.mff.cuni.cz>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>