net: mvneta: rely on open-coding updating stats for non-xdp and tx path

In oreder to avoid unnecessary instructions rely on open-coding updating
per-cpu stats in mvneta_tx/mvneta_xdp_submit_frame and mvneta_rx_hwbm
routines. This patch will be used to add xdp support to ethtool for the
mvneta driver

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>