net: dsa: mt7530: fix 10M/100M speed on MT7988 switch

Setup PMCR port register for actual speed and duplex on internally
connected PHYs of the MT7988 built-in switch. This fixes links with
speeds other than 1000M.

Fixes: 110c18bf

 ("net: dsa: mt7530: introduce driver for MT7988 built-in switch")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Acked-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Link: https://lore.kernel.org/r/a5b04dfa8256d8302f402545a51ac4c626fdba25.1706071272.git.daniel@makrotopia.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>