libata: Blacklist some Sandisk SSDs for NCQ

Sandisk SSDs SD7SN6S256G and SD8SN8U256G are regularly locking up
regularly under sustained moderate load with NCQ enabled.  Blacklist
for now.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Dave Jones <davej@codemonkey.org.uk>
Cc: stable@vger.kernel.org