ACPI: PM: Change pr_fmt() in device_pm.c

All messages printed by functions in this file either contain
the "ACPI" or "acpi" string regardless of the format, or they don't
need to contain it at all.

In the former case, the "ACPI:" string added by the format is
redundant, so drop it from there.

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