net/iucv: Add missing kernel-doc return value descriptions

Add missing return value descriptions for several functions in
net/iucv/af_iucv.c and net/iucv/iucv.c to address kernel-doc warnings.

Warnings detected with:
scripts/kernel-doc -none -Wall net/iucv/*
Warning: net/iucv/af_iucv.c:131 No description found for return value of 'iucv_msg_length'
Warning: net/iucv/af_iucv.c:150 No description found for return value of 'iucv_sock_in_state'
...

No functional change.

Reviewed-by: Aswin Karuvally <aswin@linux.ibm.com>
Reviewed-by: Alexandra Winter <wintera@linux.ibm.com>
Signed-off-by: Nagamani PV <nagamani@linux.ibm.com>
Signed-off-by: Alexandra Winter <wintera@linux.ibm.com>
Link: https://patch.msgid.link/20260330114436.2010108-1-wintera@linux.ibm.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>