selftests: firmware: skip timeout checks for kernels without user mode helper

The CONFIG_FW_LOADER_USER_HELPER is mostly disabled these days, so skip
timeout setting for these kernels.

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