platform/x86: think-lmi: Prevent underflow in index_store()

platform/x86: think-lmi: Prevent underflow in index_store()

There needs to be a check to prevent negative offsets for
setting->index.  I have reviewed this code and I think that the
"if (block->instance_count <= instance)" check in __query_block() will
prevent this from resulting in an out of bounds access.  But it's
still worth fixing.

Fixes: 640a5fa50a42 ("platform/x86: think-lmi: Opcode support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20211217071209.GF26548@kili
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>