The MDIO address of the MT7530 and MT7531 switch ICs can be configured
using bootstrap pins. However, there are only 4 possible options for the
switch itself: 7, 15, 23 and 31 (ie. only 3 and 4 can be configured, bit
0~2 are always 111). Practically all boards known as of today use the
default setting which is to have the switch respond to address 31, while
the built-in switch PHYs respond to address 0~4 in this case.