tools/power turbostat: option '-n' is ambiguous

In some cases specifying the '-n' command line argument will cause
turbostat to fail.  For instance 'turbostat -n 1' works fine; however,
'turbostat -n 1 -d' will fail.  This is the result of the first call
to getopt_long_only() where "MP" is specified as the optstring.  This can
be easily fixed by changing the optstring from "MP" to "MPn:" to remove
ambiguity between the arguments.

tools/power turbostat: option '-n' is ambiguous; possibilities: '-num_iterations' '-no-msr' '-no-perf'

Fixes: a0e86c90 ("tools/power turbostat: Add --no-perf option")

Signed-off-by: David Arcari <darcari@redhat.com>
Signed-off-by: Len Brown <len.brown@intel.com>