Eliminate some warnings from building testing.

configs/clicker2-stm32/src/stm32_mrf24j40.c

@@ -290,6 +290,7 @@ static int stm32_mrf24j40_devsetup(FAR struct stm32_priv_s *priv)
     }
 #endif
 
+  UNUSED(ret);
   return OK;
 }
 

drivers/loop/losetup.c

@@ -375,7 +375,7 @@ int losetup(FAR const char *devname, FAR const char *filename,
   FAR struct loop_struct_s *dev;
   struct stat sb;
   int ret;
-  int fd;
+  int fd = -1;
 
   /* Sanity check */