firmware: simplify CONFIG_FW_LOADER_USER_HELPER_FALLBACK further

All CONFIG_FW_LOADER_USER_HELPER_FALLBACK really is, is just a bool,
initailized at build time. Define it as such. This simplifies the
logic even further, removing now all explicit #ifdefs around the code.

Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>