net: mvneta: move rx_dropped and rx_errors in per-cpu stats

Move rx_dropped and rx_errors counters in mvneta_pcpu_stats in order to
avoid possible races updating statistics

Fixes: 562e2f46 ("net: mvneta: Improve the buffer allocation method for SWBM")
Fixes: dc35a10f ("net: mvneta: bm: add support for hardware buffer management")
Fixes: c5aff182

 ("net: mvneta: driver for Marvell Armada 370/XP network unit")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>