firmware_loader: simplfy builtin or module check

The existing check is outdated and confuses developers. Use the
already existing IS_REACHABLE() defined on kconfig.h which makes
the intention much clearer.

Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Reported-by: Borislav Petkov <bp@alien8.de>
Reported-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Ackd-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20220112160053.723795-1-mcgrof@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>