GD32: ADC workaround (#83)

(cherry picked from commit 02fd9f248189e3e801b50c920e1166daa23283bc)

Co-authored-by: Andrey Gusakov <dron0gus@gmail.com>

firmware/sampling.cpp

@@ -49,6 +49,9 @@ static void SamplingThread(void*)
     float r_2 = 0;
     float r_3 = 0;
 
+    /* GD32: Insert 20us delay after ADC enable */
+    chThdSleepMilliseconds(1);
+
     while(true)
     {
         /* TODO: run for all channels */