ACPI / util: cast data to u64 before shifting to fix sign extension

obj->buffer.pointer[i] should be cast to u64 to prevent an unintentional
sign extension.  For example, if pointer[7] is 0x80, then the value
0xffffffffff000000 is or'd into mask rather than the intended value
0xff00000000000000

Detected with static analysis by CoverityScan

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>