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>