ramips: fix mode switch gpio for AsiaRF AWM eval board

Fix a typo: gpio 21 is already used for the WPS led.  Gpio 20 is the
right one (tested) for the mode switch.  Confirmed that
/sys/kernel/debug/gpio state followed the position of the switch.

Signed-off-by: Russell Senior <russell@personaltelco.net>

SVN-Revision: 41858