tty/hvc_iucv: remove redundant NULL check

Make smatch happy and remove this warning:

drivers/tty/hvc/hvc_iucv.c:1320 hvc_iucv_init() info: redundant null check on
hvc_iucv_filter calling kfree()

Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>