s390/pai: rework paiXXX_start and paiXXX_stop functions

The PAI crypto counter and PAI NNPA counters start and stop functions
are streamlined. Move the conditions to invoke start and stop functions
to its respective function body and call them unconditionally.
The start and stop functions now determine how to proceed.
No functional change.

Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Acked-by: Mete Durlu <meted@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>