uart.c: Fix return value check for tcflush()

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

src/uart/uart.c

@@ -253,7 +253,7 @@ mraa_uart_flush(mraa_uart_context dev)
         return MRAA_ERROR_INVALID_HANDLE;
     }
 
-    if (!tcdrain(dev->fd)) {
+    if (tcdrain(dev->fd) == -1) {
         return MRAA_ERROR_FEATURE_NOT_SUPPORTED;
     }