net: dsa: mt7530: do not use SW_PHY_RST to reset MT7531 switch

According to the document MT7531 Reference Manual for Development Board
v1.0, the SW_PHY_RST bit on the SYS_CTRL register doesn't exist for
MT7531. This is likely why forcing link down on all ports is necessary for
MT7531.

Therefore, do not set SW_PHY_RST on mt7531_setup().

Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>