power: avs: qcom-cpr: fix unsigned expression compared with zero

drivers/power/avs/qcom-cpr.c:1539:5-21:
warning: unsigned expression compared with zero: drv->num_corners < 0

Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: Julia Lawall <julia.lawall@lip6.fr>
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>