s390/con3270: use proper type for tasklet function

Get rid of this warning:

drivers/s390/char/con3270.c:629:22: warning: cast between incompatible function types from ‘void (*)(struct raw3270_request *)’ to ‘void (*)(long unsigned int)’ [-Wcast-function-type]
  629 |                      (void (*)(unsigned long)) con3270_read_tasklet,
      |                      ^

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