efi_loader: GetVariable set attributes for EFI_BUFFER_TOO_SMALL

UEFI spec 2.7 erratum A leaves it undefined if Attributes should be set if
GetVariable() returns EFI_BUFFER_TOO_SMALL.

UEFI spec 2.8 defines that Attributes should be set if the return value is
either EFI_SUCCESS or EFI_BUFFER_TOO_SMALL.

Set Attributes if the return value is EFI_BUFFER_TOO_SMALL.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>