slip: fix NOHZ local_softirq_pending 08 warning

When using nanosleep() in an userspace application we get a ratelimit warning:

	NOHZ: local_softirq_pending 08

According to 481a8199142c050b72bff8a1956a49fd0a75bbe0 the problem is caused by
netif_rx() function. This patch replaces netif_rx() with netif_rx_ni() which
has to be used from process/softirq context.

Signed-off-by: Matvejchikov Ilya <matvejchikov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>