power: avs: qcom-cpr: make cpr_get_opp_hz_for_req() static

drivers/power/avs/qcom-cpr.c:1081:15:
warning: no previous prototype for ‘cpr_get_opp_hz_for_req’

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>