[S390] zcrypt: Comments and kernel-doc cleanup

Comments, which suggested to be kernel-doc but were not in the right
formatting, have been corrected. Additionally some minor cleanup in
the comments has been done.

Signed-off-by: Felix Beck <felix.beck@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>