ath79: TP-link TL-WDR4300: use fixed-link for AR8327 switch connection

ath79: TP-link TL-WDR4300: use fixed-link for AR8327 switch connection

Attaching PHY driver to the switch, while adding LEDs binding causes the
PHY driver to create additional LED instances, handled incorrectly by
the PHY driver, which are non-functional. Use fixed-link to attach the
switch driver, instead of PHY driver, to prevent that.

This has a side effect of not logging switch port up/down events in the kernel
log.

Signed-off-by: Lech Perczak <lech.perczak@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/12487
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>