drm/panfrost: simplify the return expression of panfrost_devfreq_target()

Simplify the return expression.

Signed-off-by: Qinglang Miao <miaoqinglang@huawei.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200921131021.91604-1-miaoqinglang@huawei.com