net: axienet: Switch to 64-bit RX/TX statistics

The RX and TX byte/packet statistics in this driver could be overflowed
relatively quickly on a 32-bit platform. Switch these stats to use the
u64_stats infrastructure to avoid this.

Signed-off-by: Robert Hancock <robert.hancock@calian.com>
Link: https://lore.kernel.org/r/20220829233901.3429419-1-robert.hancock@calian.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>