[S390] keyboard: integer underflow bug

The "ct" variable should be an unsigned int.  Both struct kbdiacrs
->kb_cnt and struct kbd_data ->accent_table_size are unsigned ints.

Making it signed causes a problem in KBDIACRUC because the user could
set the signed bit and cause a buffer overflow.

Cc: <stable@kernel.org>
Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>