staging: speakup: make function ser_to_dev static

The helper function ser_to_dev does not need to be in global scope, so
make it static.

Cleans up sparse warning:
"warning: symbol 'ser_to_dev' was not declared. Should it be static?"
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Okash Khawaja <okash.khawaja@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/speakup/spk_ttyio.c

@@ -21,7 +21,7 @@ struct spk_ldisc_data {
 static struct spk_synth *spk_ttyio_synth;
 static struct tty_struct *speakup_tty;
 
-int ser_to_dev(int ser, dev_t *dev_no)
+static int ser_to_dev(int ser, dev_t *dev_no)
 {
 	if (ser < 0 || ser > (255 - 64)) {
 		pr_err("speakup: Invalid ser param. Must be between 0 and 191 inclusive.\n");