s390/netiuvc: get rid of forward declarations

Move netiucv_handler to get rid of forward declarations and gcc11
compile warnings:

drivers/s390/net/netiucv.c:518:65: warning: argument 2 of type ‘u8[16]’ {aka ‘unsigned char[16]’} with mismatched bound [-Warray-parameter=]
  518 | static void netiucv_callback_connack(struct iucv_path *path, u8 ipuser[16])
      |                                                              ~~~^~~~~~~~~~
drivers/s390/net/netiucv.c:122:58: note: previously declared as ‘u8 *’ {aka ‘unsigned char *’}
  122 | static void netiucv_callback_connack(struct iucv_path *, u8 *);

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>