serial: Fix double word

Fix double word "start start" in comments.

Signed-off-by: Shaomin Deng <dengshaomin@cdjrlc.com>
Link: https://lore.kernel.org/r/20220831160934.7986-1-dengshaomin@cdjrlc.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>