PM / ACPI: Fix suspend/resume regression caused by cpuidle cleanup.

Commit e978aa7d ( cpuidle: Move
dev->last_residency update to driver enter routine; remove dev->last_state)
was  breaking suspend on laptops, as reported in the below link
	- https://lkml.org/lkml/2011/11/11/164

This was fixed in commit 3439a8da
(ACPI / cpuidle: Remove acpi_idle_suspend (to fix suspend regression)
by removing acpi_idle_suspend flag.
	- https://lkml.org/lkml/2011/11/14/74

But this did fix did not work on all systems
as Suspend/resume regression was reported on Lenovo S10-3
recently by Dave.
	- https://lkml.org/lkml/2012/5/27/115


It looked like with commit e978aa7d broke suspend and
with commit 3439a8da resume was not working with acpi_idle driver.

This patch fixes the regression that caused this issue
in the first place. acpi_idle_suspend flag is essential on
some x86 systems to prevent the cpus from going to deeper C-states
when suspend is triggered ( commit b04e7bdb )
So reverting the commit 3439a8da is essential.

By default, irqs are disabled in cpu_idle arch specific call
and re-enabled in idle state return path . During suspend,
the acpi_idle_suspend flag is set, which
prevents the cpus from going to deeper idle states,
it is essential to enabling the irqs in this return path too.

To address the suspend issue,
we were not re-enabling the interrupts while returning from
acpi_idle_enter_bm() routine if acpi_idle_suspend flag is set.
and this caused suspend failure.

In addition to the above, to improve the readability of the code,
return of -ENIVAL is replaced with -EBUSY in acpi_idle_suspend
return path. Implying that the system is currently busy when suspend
is in progress, which prevents the cpus from entering deeper C-states.

Reported-and-Tested-by: Dav Hansen <dave@linux.vnet.ibm.com>
Tested-by: Preeti Murthy <preeti@linux.vnet.ibm.com>
Signed-off-by: Deepthi Dharwar <deepthi@linux.vnet.ibm.com>
Reviewed-by: Srivatsa S Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>