s390/pai: cleanup event initialization

s390/pai: cleanup event initialization

Setting event::hw.last_tag to zero is not necessary. The memory
for each event is dynamically allocated by the kernel common code and
initialized to zero already.  Remove this unnecessary assignment.
Move the comment to function paicrypt_start() for clarification.

Suggested-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
Acked-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>