x86: cpufeature: fix SMX flag

Impact: "smx" flags showed as "safer" in /proc/cpuinfo

The SMX feature flag is the so-called "safer mode"... I had put that
in quotes, but that flagged it as a cpuinfo flag name.  Remove the
quotes to correct the flag name in /proc/cpuinfo.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>