[config-setup]: Fix a bug in checking if updategraph is enabled (#7093)

Encounter error during "config-setup boot" if the updategraph is enabled.

How I did it
Correct the code inside the config-setup script.
Remove the space between the assignment operator.

How to verify it
Remove the /etc/sonic/config_db.json and reboot the device.
Originally, it will return following error after boot up.
rv: command not found
After modification, it can correctly parse the status of updategraph without error.

files/image_config/config-setup/config-setup

@@ -158,7 +158,7 @@ updategraph_is_enabled()
     rv=1
     if [ -e ${UPDATEGRAPH_CONF} ]; then
         updategraph_mode=$(grep enabled ${UPDATEGRAPH_CONF} | head -n 1 | cut -f2 -d=)
-        [ "${updategraph_mode}" = "true" ] && rv = 0
+        [ "${updategraph_mode}" = "true" ] && rv=0
     fi
     return $rv
 }