Fix URE_WDT6_SET_MODE value in the register definition.
Both linux and u-boot sources for RTL8152 driver has this value.
RTL8152 USB ethernet is used in NanoPI R1 board as second ethernet.
This fixes for me RTL8152 USB ethernet not detected problem after
reboot on NanoPI R1 board.

Both NetBSD and OpenBSD have a wrong value so far.