efi_loader: update SetVariable attribute check

UEFI specification v2.10 says that
EFI_VARIABLE_AUTHENTICATED_WRITE_ACCESS is deprecated and
EFI_UNSUPPORTED should be returned in SetVariable variable service.
Current implementation returns EFI_INVALID_PARAMETER,
let's fix the return value.

Together with above change, this commit also updates the SetVariable
attribute check to be aligned with the EDK2 reference implementation.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>