can: mcp251x: fix NOHZ local_softirq_pending 08 warning

This patch replaces netif_rx() with netif_rx_ni() which has to be used
from the threaded interrupt i.e. process context context.

Thanks to Christian Pellegrin for pointing at the right fix:
481a8199142c050b72bff8a1956a49fd0a75bbe0 by Oliver Hartkopp.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Acked-by: Wolfgang Grandegger <wg@grandegger.com>