net: dsa: lantiq_gswip: Add 200ms assert delay

The delay is especially needed by the xRX300 and xRX330 SoCs. Without
this patch, some phys are sometimes not properly detected.

The patch was tested on BT Home Hub 5A and D-Link DWR-966.

Fixes: a09d042b ("net: dsa: lantiq: allow to use all GPHYs on xRX300 and xRX330")
Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: David S. Miller <davem@davemloft.net>