ACPI: increase size of acpi_bus_id[]

Previously [5], now [8].

sprintf(acpi_device_bid(device), "CPU%X", cpu_id)
now looks better on systems with more than 0xFF processors.

Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>