isp(4): Fix reading NVRAM contents for 28xx based devices

Use correct NVRAM address for ISP28xx based HBAs to read NVRAM contents.

WWPN/WWNN and framesize are correctly read from NVRAM now.

PR: 271062
Reviewed by: imp, mav
Sponsored by: Technical University of Munich
Pull Request: https://github.com/freebsd/freebsd-src/pull/726

(cherry picked from commit f76f7fefa1bb5dc2ec0c9e44740faac0b46dac25)