cpupower: mperf monitor: fix output in MAX_FREQ_SYSFS mode

There is clearly wrong output when mperf monitor runs in MAX_FREQ_SYSFS mode:
average frequency shows in kHz unit (despite the intended output to be in MHz),
and percentages for C state information are all wrong (including high/negative
values shown).

The problem is that the max_frequency read on initialization isn't used where it
should have been used on mperf_get_count_percent (to estimate the number of
ticks in the given time period), and the value we read from sysfs is in kHz, so
we must divide it to get the MHz value to use in current calculations.

While at it, also I fixed another small issues in the debug output of
max_frequency value in mperf_get_count_freq.

Signed-off-by: Herton R. Krzesinski <herton@redhat.com>
Acked-by: Thomas Renninger <trenn@suse.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>