OvmfPkg/EnrollDefaultKeys: clean up Base64Decode() retval handling

OvmfPkg/EnrollDefaultKeys: clean up Base64Decode() retval handling

Since commit 35e242b698cd ("MdePkg/BaseLib: rewrite Base64Decode()",
2019-07-16), Base64Decode() guarantees that DestinationSize is larger on
output than it was on input if RETURN_BUFFER_TOO_SMALL is returned. Clean
up the retval handling for the first Base64Decode() call in
EnrollDefaultKeys, which used to work around the ambiguity in the previous
Base64Decode() interface contract.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1981
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>