ACPI: event: replace strcpy() by strscpy()

The strcpy() function is being deprecated. Replace it by the safer
strscpy() and fix the following Coverity warnings:

"You might overrun the 15-character fixed-size string event->bus_id
by copying bus_id without checking the length."

"You might overrun the 20-character fixed-size string event->device_class
by copying device_class without checking the length."

Addresses-Coverity-ID: 139001 ("Copy into fixed size buffer")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>