[S390] sclp vt220: fix compile warning

get rid of this one:

  CC      drivers/s390/char/sclp_vt220.o
drivers/s390/char/sclp_vt220.c:588: warning: '__sclp_vt220_flush_buffer' defined but not used

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>