x86, vmi: TSC going backwards check in vmi clocksource, cleanup

clean up vmi_read_cycles to use max()

Reported-b: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Alok N Kataria <akataria@vmware.com>
Cc: Zach Amsden <zach@vmware.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>