FF: replace broken #if Q_LITTLE_ENDIAN

Q_LITTLE_ENDIAN was used incorrectly, it should have been
"#if Q_BYTE_ORDER == Q_LITTLE_ENDIAN"

Use qFromBigEndian since it is more explicit.

Fixes 6d5e61f0b0

src/imageformats/ff.cpp

@@ -176,11 +176,7 @@ bool FFHandler::read(QImage *image)
             qCWarning(LOG_FFPLUGIN) << "FFHandler::read() error while reading image scanline";
             return false;
         }
-#if Q_LITTLE_ENDIAN
-        for (auto i = 0; i < size; i += 2) {
-            std::swap(line[i], line[i + 1]);
-        }
-#endif
+        qFromBigEndian<quint16>(line, size / 2, line);
     }
 
     img.setColorSpace(QColorSpace(QColorSpace::SRgb));