s390/diag: make __diag8c_tmp_amode31 static

Get rid of this sparse warning:

arch/s390/kernel/diag.c:69:29: warning: symbol '__diag8c_tmp_amode31' was not declared. Should it be static?

Fixes: fbaee746 ("s390/tty3270: add support for diag 8c")
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>