x86/spectre: Add missing family 6 check to microcode check

The check for Spectre microcodes does not check for family 6, only the
model numbers.

Add a family 6 check to avoid ambiguity with other families.

Fixes: a5b29663

 ("x86/cpufeature: Blacklist SPEC_CTRL/PRED_CMD on early Spectre v2 microcodes")
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: x86@kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20180824170351.34874-2-andi@firstfloor.org