efi_loader: call EFI_EXIT in efi_copy_mem, efi_set_mem

EFI_ENTRY and EFI_EXIT calls must match.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Alexander Graf <agraf@suse.de>