drivers/staging/speakup/main: fixed jiffie comparison

speakup_key() used manual comparison of jiffies to determine the time
since the last keypress, replaced it with time_after()

Signed-off-by: Robin Schroer <sulamiification@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>