firmware_loader: move CONFIG_FW_LOADER_USER_HELPER switch to Makefile

The whole code of fallback_table.c is surrounded by #ifdef of
CONFIG_FW_LOADER_USER_HELPER.

Move the CONFIG_FW_LOADER_USER_HELPER switch to Makefile so that
it is not compiled at all when this CONFIG option is disabled.

I also removed the confusing comment, "Module or buit-in [sic]".
CONFIG_FW_LOADER_USER_HELPER is a boolean option.
(If it were a module, CONFIG_FW_LOADER_USER_HELPER_MODULE would
be defined instead.)

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>