GPS driver: use FIONREAD on NuttX only

src/drivers/gps/gps.cpp

@@ -402,6 +402,7 @@ int GPS::pollOrRead(uint8_t *buf, size_t buf_length, int timeout)
 			 * If we have all requested data available, read it without waiting.
 			 * If more bytes are available, we'll go back to poll() again.
 			 */
+#ifdef __PX4_NUTTX
 			int err = 0, bytesAvailable = 0;
 			err = ioctl(_serial_fd, FIONREAD, (unsigned long)&bytesAvailable);
 
@@ -409,6 +410,10 @@ int GPS::pollOrRead(uint8_t *buf, size_t buf_length, int timeout)
 				usleep(GPS_WAIT_BEFORE_READ * 1000);
 			}
 
+#else
+			usleep(GPS_WAIT_BEFORE_READ * 1000);
+#endif
+
 			ret = ::read(_serial_fd, buf, buf_length);
 
 		} else {