net: mt7531: only do PLL once after the reset

Move the PLL init of the switch out of the pad configuration of the port
6 (usally cpu port).

Fix a unidirectional 100 mbit limitation on 1 gbit or 2.5 gbit links for
outbound traffic on port 5 or port 6.

Fixes: c288575f ("net: dsa: mt7530: Add the support of MT7531 switch")
Cc: stable@vger.kernel.org
Signed-off-by: Alexander Couzens <lynxis@fe80.eu>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>