cpuidle,intel_idle: Fix CPUIDLE_FLAG_IRQ_ENABLE

Commit c227233a ("intel_idle: enable interrupts before C1 on
Xeons") wrecked intel_idle in two ways:

 - must not have tracing in idle functions
 - must return with IRQs disabled

Additionally, it added a branch for no good reason.

Fixes: c227233a

 ("intel_idle: enable interrupts before C1 on Xeons")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
[ rjw: Moved the intel_idle() kerneldoc comment next to the function ]
Cc: 5.16+ <stable@vger.kernel.org> # 5.16+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>