Fix the following W=1 kernel build warning(s):

drivers/acpi/scan.c: In function ‘acpi_bus_get_wakeup_device_flags’:
drivers/acpi/scan.c:902:43: warning: suggest braces around empty body in
an ‘if’ statement [-Wempty-body]

by using pr_debug() to instead of the ACPI_DEBUG_PRINT() macro, which
should only be used by ACPICA code, to print a debug message.
Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>