s390/zcrypt: simplify cca_findcard2 loop code

Instead of two times go through the list of available AP devices
(which may be up to 256 * 256 entries) this patch reworks the code do
only run through once.  The price is instead of reporting all possible
devices to the caller only the first 256 devices are collected.
However, having to choose from 256 AP devices is plenty of resources
and should fulfill the caller's requirements. On the other side
the loop code is much simplier and more easy to maintain.

Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Reviewed-by: Ingo Franzki <ifranzki@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>