When spanning tree protocol is allowed in bridge settings, forward delay
value is set as well (default is 0 if omitted). Until now, there was no
check for delay value validity. Delay makes sense only as a positive
numerical value.

Note: However, even if you provide positive  numerical value, brctl
utility only uses values from range <2,30>, so the number provided can
be modified (kernel most likely) to fall within this range.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1125764