speakup: Fix 8bit characters from direct synth

When userland echoes 8bit characters to /dev/synth with e.g.

echo -e '\xe9' > /dev/synth

synth_write would get characters beyond 0x7f, and thus negative when
char is signed.  When given to synth_buffer_add which takes a u16, this
would sign-extend and produce a U+ffxy character rather than U+xy.
Users thus get garbled text instead of accents in their output.

Let's fix this by making sure that we read unsigned characters.
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Fixes: 89fc2ae8 ("speakup: extend synth buffer to 16bit unicode characters")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20240204155736.2oh4ot7tiaa2wpbh@beginSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/accessibility/speakup/synth.c

@@ -208,8 +208,10 @@ void spk_do_flush(void)
 	wake_up_process(speakup_task);
 }
 
-void synth_write(const char *buf, size_t count)
+void synth_write(const char *_buf, size_t count)
 {
+	const unsigned char *buf = (const unsigned char *) _buf;
+
 	while (count--)
 		synth_buffer_add(*buf++);
 	synth_start();