As EFI_ERROR is not a UEFI result code but a local invention only
intended to allow get_dram_base() to signal failure, we should not use
it elsewhere.

Replace with EFI_LOAD_ERROR.
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>