l3gd20: checking status only makes sense if we have DRDY

it makes no sense on the external SPI bus

src/drivers/l3gd20/l3gd20.cpp

@@ -972,7 +972,7 @@ L3GD20::measure()
 	transfer((uint8_t *)&raw_report, (uint8_t *)&raw_report, sizeof(raw_report));
 
 #if L3GD20_USE_DRDY
-        if ((raw_report.status & 0xF) != 0xF) {
+        if (_bus == PX4_SPI_BUS_SENSORS && (raw_report.status & 0xF) != 0xF) {
             /*
               we waited for DRDY, but did not see DRDY on all axes
               when we captured. That means a transfer error of some sort