s390/ap_bus: use and-mask instead of a cast

Let's get rid of another sparse false positive:

drivers/s390/crypto/ap_bus.c:416:64: warning:
  cast truncates bits from constant value (102030405060708 becomes 5060708)

So instead of using a cast let's use an and-mask.
That way sparse remains silent and one doesn't always have to check
if this is a valid warning/bug or just a false positive.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>