ath79: set mib-poll-interval on mdio0 attached ar83xx switch

ath79: set mib-poll-interval on mdio0 attached ar83xx 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 have
ar83xx switch connected to mdio0 bus, as the same value was set for
built in switches in 443fc9ac35 ("ath79: use ar8216 for builtin
switch").

Some measurements performed on TP-Link Archer C7-v5:

 mib-type=0, mib-poll-interval=500ms (10s pidstat)

  Average:  %usr %system  %guest   %wait    %CPU   CPU  Command
  Average:  0.00    1.93    0.00    0.00    1.93     -  kworker/0:2

  iperf3 (30s): 334 Mbits/sec

 mib-type=0, mib-poll-interval=2s (10s pidstat)

  Average:  %usr %system  %guest   %wait    %CPU   CPU  Command
  Average:  0.00    1.14    0.00    0.00    1.14     -  kworker/0:2

  iperf3 (30s): 334 Mbits/sec

So it seems like we get 4x faster LED refresh rate for additional 0.8%
CPU load.

Signed-off-by: Petr Štetiar <ynezz@true.cz>