[X86] Fix warning in cpu detection due to unsigned comparison

a15532d7 landed a patch that added
support for detecting more AMD znver2 CPUs and cleaned up some of the
surrounding code, including the znver3 detection. Since one model group
is 00h-0fh, I adjusted the check to include checking if the value is
greater than zero. Since the value is unsigned, this is always true and
gcc warns on it. This patch removes the comparison with zero to get rid
of the compiler warning.