generic: make switch_port_stats tx/rx_bytes long long

generic: make switch_port_stats tx/rx_bytes long long

This generic structure defines tx_bytes and rx_bytes as unsigned long (u32),
while several devices would typically report unsigned long long (u64).

The code can work as is, but there's a chance that with a sufficiently fast
interface the overflow might happen too fast to be correctly noticed by the
consumers of this data.

This patch makes both field unsigned long long and updates the only known
consumer of this data: swconfig_leds.c

Signed-off-by: Thibaut VARENE <hacks@slashdirt.org>