net: dsa: mt7530: set PLL frequency and trgmii only when trgmii is used

[ Upstream commit 0b086d76 ]

As my testing on the MCM MT7530 switch on MT7621 SoC shows, setting the PLL
frequency does not affect MII modes other than trgmii on port 5 and port 6.
So the assumption is that the operation here called "setting the PLL
frequency" actually sets the frequency of the TRGMII TX clock.

Make it so that it and the rest of the trgmii setup run only when the
trgmii mode is used.

Tested rgmii and trgmii modes of port 6 on MCM MT7530 on MT7621AT Unielec
U7621-06 and standalone MT7530 on MT7623NI Bananapi BPI-R2.

Fixes: b8f126a8

 ("net-next: dsa: add dsa support for Mediatek MT7530 switch")
Tested-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Link: https://lore.kernel.org/r/20230310073338.5836-2-arinc.unal@arinc9.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>