s390/crypto: cleanup cpacf function codes

Use a separate define for the decryption modifier bit instead of
duplicating the function codes for encryption / decrypton.
In addition use an unsigned type for the function code.

Reviewed-by: Harald Freudenberger <freude@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>