[S390] sclp_vt220: set initial terminal window size

When opening a SCLP VT220 terminal, the terminal window size is not
initialized (defaults to zero).
Since the SCLP VT220 terminal supports only 80x24, explicitly set
the window size to prevent (n)curses applications from guessing
the default setting.

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