firmware_loader: move fw_fallback_config to a private kernel symbol namespace

Take advantage of the new kernel symbol namespacing functionality, and
export the fw_fallback_config symbol only to a new private firmware loader
namespace. This would prevent misuses from other drivers and makes it clear
the goal is to keep this private to the firmware loader only.

It should also make it clearer for folks git grep'ing for users of
the symbol that this exported symbol is private, and prevent future
accidental removals of the exported symbol.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Link: https://lore.kernel.org/r/20200424184916.22843-2-mcgrof@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>