realtek: dsa: Clarify statistic port iterator variable

realtek: dsa: Clarify statistic port iterator variable

The functions iterating through the port statistic/counter (for
initialization or polling) use the generic name "i" for the iterator. This
makes reading the actual body of the loop cumbersome because it is not
clear that various parameters of functions are about a ports.

Suggested-by: Felix Baumann <felix.bau@gmx.de>
Signed-off-by: Sven Eckelmann <se@simonwunderlich.de>
Link: https://github.com/openwrt/openwrt/pull/20631
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>