power: avs: qcom-cpr: remove set but unused variable

drivers/power/avs/qcom-cpr.c:896:35:
warning: variable ‘prev’ set but not used

Detected when running make with W=1.

Signed-off-by: Niklas Cassel <nks@flawful.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>