LinuxKPI: firmware: change order filenames trying to load firmware

Given we have the support in main and stable/14 to load plain firmware
files from /boot/firmware/ make use of that. Switch the order to
try the original unmangled filename and path first before trying any
replacements.
This reduces time and console noise (especially under bootverbose).

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D47682