tty: hvc-iucv: Make use of iucv_alloc_device()

Make use of iucv_alloc_device() to get rid of quite some code.

Acked-by: Alexandra Winter <wintera@linux.ibm.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20240506194454.1160315-6-hca@linux.ibm.com


Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>