dvfs: Fix set_rate frequency multiplier

The recent change of frequency unit from Hz to kHz has left one
set_rate call with the previous units, resulting in a failed frequency
change.

Align the remaining set_rate with the correct frequency unit.

Change-Id: I850e4433fe9b9f56a3167461afca482e3f1dcf88
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>