isp(4): Remove redundant functions for reading data from FLT and flash

Rename isp_rd_2400_nvram to isp_rd_2xxx_flash.
Cleanup some leftovers.
Hide all output regarding FLT parsing behind ISP_LOGDEBUG0.

Thanks to imp@ and mav@ for reviewing and commenting.

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 295fd9c1caeb42a4c93005080320d011bf01e4c0)