Bugfix: On error, return the actual error code in sanei_magic_rotate

Up to now, on failure the sanei_magic_rotate function returned a
success status.  This fixes the following GCC warning:

warning: variable 'ret' set but not used

sanei/sanei_magic.c

@@ -710,7 +710,7 @@ sanei_magic_rotate (SANE_Parameters * params, SANE_Byte * buffer,
 
   DBG(10,"sanei_magic_rotate: finish\n");
 
-  return 0;
+  return ret;
 }
 
 SANE_Status