speakup: Fix sizeof() vs ARRAY_SIZE() bug

speakup: Fix sizeof() vs ARRAY_SIZE() bug

The "buf" pointer is an array of u16 values.  This code should be
using ARRAY_SIZE() (which is 256) instead of sizeof() (which is 512),
otherwise it can the still got out of bounds.

Fixes: c8d2f34ea96e ("speakup: Avoid crash on very long word")
Cc: stable@vger.kernel.org
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Link: https://lore.kernel.org/r/d16f67d2-fd0a-4d45-adac-75ddd11001aa@moroto.mountain
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>