cpufreq: intel_pstate: Get rid of unnecessary READ_ONCE() annotations

Drop two redundant checks involving READ_ONCE() from notify_hwp_interrupt()
and make it check hwp_active without READ_ONCE() which is not necessary,
because that variable is only set once during the early initialization of
the driver.

In order to make that clear, annotate hwp_active with __ro_after_init.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>