LinuxKPI: treat firmware file names more lenient

A lot of firmware files have a "-" in the name. That "-" is a problem
when dealing with shell variables or loader (e.g., auto-loading .ko).
It may thus often be convenient to generate firmware kernel object files
with s/-/_/g in the name. In order to automatically find them from
drivers using LinuxKPI also substitue the '-' for a '_' like we do
for '/' and '.' already.

Reviewed-by: hselasky, manu (ok)
MFC-after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D29514