perf intel-pt: Fix MTC timing after overflow

On some platforms, overflows will clear before MTC wraparound, and there
is no following TSC/TMA packet. In that case the previous TMA is valid.
Since there will be a valid TMA either way, stop setting 'have_tma' to
false upon overflow.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: stable@vger.kernel.org
Link: http://lkml.kernel.org/r/1527762225-26024-4-git-send-email-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>