s390/cpacf: get rid of register asm

Using register asm statements has been proven to be very error prone,
especially when using code instrumentation where gcc may add function
calls, which clobbers register contents in an unexpected way.

Therefore get rid of register asm statements in cpacf code, and make
sure this bug class cannot happen.
Reviewed-by: Patrick Steuer <patrick.steuer@de.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>