tty: serial: Fix spelling mistake "Asychronous" -> "Asynchronous"

There is a spelling mistake in the Kconfig text. Fix it.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20210719102421.14813-1-colin.king@canonical.com


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