cpufreq: intel_pstate: Simplify spinlock locking

Because intel_pstate_enable/disable_hwp_interrupt() are only called from
thread context, they need not save the IRQ flags when using a spinlock
as interrupts are guaranteed to be enabled when they run, so make them
use spin_lock/unlock_irq().

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