Fix code repetition

src/main.c

@@ -11,14 +11,10 @@ int main(void)
 
 	for (i = 1;; i++) {
 		if (i >= 99999) i = 1;
-
-		if (USART_IsDataAvailable()) {
-			while ((ch = USART_GetChar())) {
-				CMD_Parse(ch);
-			}
-		}
-
 		USART_Printf("[CORE] Fetching sensors #%05lu...\r\n", i);
+		while ((ch = USART_GetChar())) {
+			CMD_Parse(ch);
+		}
 
 		Sleep(3000);
 	}

src/serial.c

@@ -34,7 +34,7 @@ char USART_GetChar(void)
 {
 	unsigned char newtail;
 
-	if (rxhead == rxtail) {
+	if (!USART_IsDataAvailable()) {
 		return '\0'; // Non-blocking
 	}