ACPI: event: Use pr_*() macros to replace printk()

Introduce pr_fmt() and replace direct printk() invocation with
the matching pr_*() call to prepare for removing PREFIX.

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