databases/mysql56-server: Fix rc-script ${name}_limits conflict for post r328331
After r328331 changes in head/etc/rc.subr, having "NO" in mysql_limits
goes into "limits" command params as is.
So this patch substitutes the "NO" appropriately when needed.

PR: 225657
Reported by: robbak@gmail.com
Reviewed by: Rainer Hurling <rhurlin@gwdg.de>
Sponsored by: Netzkommune GmbH