net: dsa: mt7530: fix support for MT7531BE

There are two variants of the MT7531 switch IC which got different
features (and pins) regarding port 5:
 * MT7531AE: SGMII/1000Base-X/2500Base-X SerDes PCS
 * MT7531BE: RGMII

Moving the creation of the SerDes PCS from mt753x_setup to mt7530_probe
with commit 6de28522 ("net: dsa: mt7530: move SGMII PCS creation
to mt7530_probe function") works fine for MT7531AE which got two
instances of mtk-pcs-lynxi, however, MT7531BE requires mt7531_pll_setup
to setup clocks before the single PCS on port 6 (usually used as CPU
port) starts to work and hence the PCS creation failed on MT7531BE.

Fix this by introducing a pointer to mt7531_create_sgmii function in
struct mt7530_priv and call it again at the end of mt753x_setup like it
was before commit 6de28522 ("net: dsa: mt7530: move SGMII PCS
creation to mt7530_probe function").

Fixes: 6de28522

 ("net: dsa: mt7530: move SGMII PCS creation to mt7530_probe function")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Acked-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Link: https://lore.kernel.org/r/ZDvlLhhqheobUvOK@makrotopia.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>