tools/power turbostat: Fix offset overflow issue in index converting

The idx_to_offset() function returns type int (32-bit signed), but
MSR_PKG_ENERGY_STAT is u32 and would be interpreted as a negative number.
The end result is that it hits the if (offset < 0) check in update_msr_sum()
which prevents the timer callback from updating the stat in the background when
long durations are used. The similar issue exists in offset_to_idx() and
update_msr_sum(). Fix this issue by converting the 'int' to 'off_t' accordingly.

Fixes: 9972d5d8 ("tools/power turbostat: Enable accumulate RAPL display")
Signed-off-by: Calvin Walton <calvin.walton@kepstin.ca>
Signed-off-by: Len Brown <len.brown@intel.com>