AP_HAL: fix another RCInput compile error

libraries/AP_HAL_AVR/examples/RCPassthroughTest/RCPassthroughTest.pde

@@ -27,7 +27,7 @@ void multiread(AP_HAL::RCInput* in, uint16_t* channels) {
 void individualread(AP_HAL::RCInput* in, uint16_t* channels) {
     /* individual channel read method: */
     uint8_t valid;
-    valid = in->valid();
+    valid = in->valid_channels();
     for (int i = 0; i < 8; i++) {
         channels[i] = in->read(i);
     }