On OpenWRT this causes the kernel to completely ignore the PCIe interface and everything attached to it, so only the onchip 2.4GHz radio will work. On the original firmware it may cause the unit to get stuck in a reboot loop.
If this happens, just power the unit off for about 30 seconds. In fact it might be a good idea to power cycle the unit before OpenWRT's initial boot. See this thread for more information.