hwpmc: improve diagnostic messages for invalid tunables

Replace printf() with log(LOG_WARNING, ...) in pmclog_initialize()
so that tunable validation failures are visible in dmesg and
/var/log/messages rather than only on the early console.

Also improve the messages to report both the invalid value and the
default it resets to, making it easier for users to understand why
their tunable was ignored.

While here, adjust some whitespacing/style.

Reviewed by: Ali Mashtizadeh <ali@mashtizadeh.com>, mhorne
MFC after: 1 week
Sponsored by: NLINK (nlink.com.br)
Differential Revision: https://reviews.freebsd.org/D56029