speakup: replace sprintf() by scnprintf()

Replace sprintf() by scnprintf() in order to avoid buffer overflows.

Signed-off-by: Salah Triki <salah.triki@gmail.com>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Link: https://lore.kernel.org/r/20210630224248.2iq6o6krecx4cz5j@begin


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>