s390/sclp_vt220: fix unused function warning

s390/sclp_vt220: fix unused function warning

When CONFIG_SCLP_VT220_TTY=y and CONFIG_SCLP_VT220_CONSOLE is not set:

../drivers/s390/char/sclp_vt220.c:771:13: warning: '__sclp_vt220_flush_buffer' defined but not used [-Wunused-function]
  771 | static void __sclp_vt220_flush_buffer(void)

so move this function inside the #ifdef block where it is used.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Link: https://lore.kernel.org/r/20210927215647.11506-1-rdunlap@infradead.org
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>