shell/sc_can: fix uninitialized warning

When compiled for `hifive1` board with `gcc-7.2.0` this warning was raised:

'ret' may be used uninitialized in this function [-Werror=maybe-uninitialized]

sys/shell/commands/sc_can.c

@@ -83,7 +83,7 @@ static int _dump(int argc, char **argv)
         return 0;
     }
 
-    int ret;
+    int ret = -1;
     struct can_filter filters[SC_CAN_MAX_FILTERS];
     conn_can_raw_t conn;
     struct can_frame frame;