ispfw(4): Update and add firmware

Update 25xx firmware: version 8.8.207
Add 26xx firmware: version 8.8.231
Add 27xx firmware: version 9.12.0
Add 28xx firmware: version 9.12.1

Remove BUGS section from manpage as we now have firmware for
all supported controllers.

PR: 273263
MFC after: 1 month
Pull Request: https://github.com/freebsd/freebsd-src/pull/877
Sponsored by: Technical University of Munich