tools/power turbostat: fix error case overflow read of slm_freq_table[]

When i >= SLM_BCLK_FREQS, the frequency read from the slm_freq_table
is off the end of the array because msr is set to 3 rather than the
actual array index i.  Set i to 3 rather than msr to fix this.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Len Brown <len.brown@intel.com>