mac802154: fix NOHZ local_softirq_pending 08 warning

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

NOHZ: local_softirq_pending 08

for 10 times.

This patch replaces netif_rx() with netif_rx_ni() which has
to be used from process/softirq context.
The process/softirq context will be called from fakelb driver.

See linux-kernel commit 481a8199 for similar fix.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>