net: dsa: mt7530: move setting ssc_delta to PHY_INTERFACE_MODE_TRGMII case

Move setting the ssc_delta variable to under the PHY_INTERFACE_MODE_TRGMII
case as it's only needed when trgmii is used.

Fixes: b8f126a8

 ("net-next: dsa: add dsa support for Mediatek MT7530 switch")
Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Link: https://lore.kernel.org/r/20230320190520.124513-3-arinc.unal@arinc9.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>