libata-acpi: don't call sleeping function from invalid context

The problem is introduced by commit
664d080c.

acpi_evaluate_integer is a sleeping function,
and it should not be called with spin_lock_irqsave.
https://bugzilla.redhat.com/show_bug.cgi?id=451399



Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>