ar71xx: set mib-poll-interval on devices with ar8xxx switch

ar71xx: set mib-poll-interval on devices with ar8xxx switch

Commit "generic: ar8216: add mib_poll_interval switch attribute" sets
mib-poll-interval as disabled by default (was set to 2s), so it makes
switch LEDs trigger disfunctional on devices which don't have
mib-poll-interval set.

So this patch sets mib-poll-interval to 500ms on devices which are using
ar8xxx switch LEDs trigger, as the same value was set for built in
switches in 443fc9ac35 ("ath79: use ar8216 for builtin switch").

Signed-off-by: Petr Štetiar <ynezz@true.cz>
Signed-off-by: Chuanhong Guo <gch981213@gmail.com>