hwpmc: set default rate if event description lacks one / filter rate against misuse

Not all event descriptions have a sample rate (such as inst_retired.any)
this will restore the legacy behavior of using 65536 in that case. It also
prevents accidental API misuse that could lead to panic.

PR: 230985
Reported by: markj
Reviewed by: markj
Approved by: re (gjb)
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D16958