staging/speakup: fix checkpatch.pl warning in speak_char()

correct the following warning from checkpatch.pl:-

WARNING: Prefer using '"%s...", __func__' to using 'speak_char', this
function's name, in a string

Signed-off-by: Justin Skists <j.skists@gmail.com>
Acked-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>