bridgestp: validate timer values in config BPDU

IEEE Std 802.1D-2004 Section 17.14 defines permitted ranges for timers.
Incoming BPDU messages should be checked against the permitted ranges.
The rest of 17.14 appears to be enforced already.

PR: 254924
Reviewed by: kp, donner
Differential Revision: https://reviews.freebsd.org/D29782