generic: provide get_port_stats() on adm6996 switches

This patch provides a generic switch_dev_ops 'get_port_stats()' callback by
taping into the relevant port MIB counters.

This callback is used by swconfig_leds led trigger to blink LEDs with port
network traffic.

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