s390/tty3270: remove function callback casts

In an effort to enable -Wcast-function-type in the top-level Makefile to
support Control Flow Integrity builds, remove all the function callback
casts.

To do this modify the function prototypes accordingly.

Signed-off-by: Oscar Carter <oscar.carter@gmx.com>
Message-Id: <20200627125417.18887-1-oscar.carter@gmx.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
[heiko.carstens@de.ibm.com: coding style changes]
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>