PM: QoS: Adjust pm_qos_request() signature and reorder pm_qos.h

Change the return type of pm_qos_request() to be the same as the
one of pm_qos_read_value() called by it internally and stop exporting
it to modules (because its only caller, cpuidle, is not modular).

Also move the pm_qos_read_value() header away from the CPU latency
QoS API function headers in pm_qos.h (because it technically does
not belong to that API).

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Amit Kucheria <amit.kucheria@linaro.org>
Tested-by: Amit Kucheria <amit.kucheria@linaro.org>