s390/netiucv: Fix return type of netiucv_tx()
This is an automated ID intended to aid in discovery of potential security vulnerabilities. The actual impact and attack plausibility have not yet been proven. This ID is fixed in Linux Kernel version v5.10.163 by commit 3ac0217ca9186c2f9af9a0113a331a42aa847894. For more details please see the references link.