cpufreq: Change order of online() CB and policy->cpus modification

From a state where all policy->related_cpus are offline, putting one
of the policy's CPU back online re-activates the policy by:
 1. Calling cpufreq_driver->online()
 2. Setting the CPU in policy->cpus

qcom_cpufreq_hw_cpu_online() makes use of policy->cpus. Thus 1. and 2.
should be inverted to avoid having a policy->cpus empty. The
qcom-cpufreq-hw is the only driver affected by this.

Fixes: a1eb080a ("cpufreq: qcom-hw: provide online/offline operations")
Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>