CTL_STATUS_t ctl_mac_mii_deferred_read(CTL_NET_INTERFACE_t *net, int reg);
ctl_mac_mii_deferred_read is a wrapper around the mii_deferred_read_fn member of the network MAC driver.
The valid range for devAddr is 0 through 31 and needs to match the PHY chip's physical address, which is typically set on the PHY hardware using strapping pins. See your PHY chip's datasheet for valid values of reg.
You can retrieve the result of the deferred read using ctl_mac_mii_deferred_read_result.