[refactor] Minor changes

Author: devkapilbansal

openwisp-monitoring/files/monitoring.agent

@@ -347,9 +347,9 @@ main() {
 	done
 
 	INTERVAL=${INTERVAL:-300}
-	REGISTRATION_INTERVAL=$((INTERVAL / 10))
 	INTERVAL="$(time_to_seconds "$INTERVAL")"
 	[ -z "$INTERVAL" ] && echoerr "Interval is invalid. Use time value(eg: '10', '2m', '3h', '1d')"
+	REGISTRATION_INTERVAL=$((INTERVAL / 10))
 	VERBOSE_MODE=${VERBOSE_MODE:-0}
 	BOOTUP_DELAY=${BOOTUP_DELAY:-10}
 	TMP_DIR="/tmp/openwisp/monitoring"