ipmi(4): Limit maximum watchdog pre-timeout interval.

Previous code by default setting pre-timeout interval to 120 seconds
made impossible to set timeout interval below that, resulting in error
0xcc (Invalid data field in Request) at least on Supermicro boards.

To fix that limit maximum pre-timeout interval to ~1/4 of the timeout
interval, that sounds like a reasonable default: not too short to fire
too late, but also not too long to give many false reports.

MFC after: 2 weeks