x86, alternatives: Fix one more open-coded 8-bit alternative number

Fix a missing case of an 8-bit alternative number, buried inside an
assembly macro.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Reported-by: Yinghai Lu <yinhai@kernel.org>
Cc: Suresh Siddha <suresh.b.siddha@intel.com>
LKML-Reference: <4C3BDDA3.2060900@kernel.org>