efi/fdt: fix panic when no valid fdt found

setup_arch() would invoke efi_init()->efi_get_fdt_params(). If no
valid fdt found then initial_boot_params will be null. So we
should stop further fdt processing here. I encountered this
issue on risc-v.

Signed-off-by: Changbin Du <changbin.du@gmail.com>
Fixes: b91540d5

 ("RISC-V: Add EFI runtime services")
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>