serial-dmm: only do bytes to text conversion at higher log levels

Check the current log level before starting the potentially expensive
bytes to text conversion for a debug message (dump DMM packets).

src/hardware/serial-dmm/protocol.c

@@ -31,6 +31,9 @@ static void log_dmm_packet(const uint8_t *buf, size_t len)
 {
 	GString *text;
 
+	if (sr_log_loglevel_get() < SR_LOG_DBG)
+		return;
+
 	text = sr_hexdump_new(buf, len);
 	sr_dbg("DMM packet: %s", text->str);
 	sr_hexdump_free(text);