speakup_dummy: log about characters received by the dummy driver

The dummy speakup driver is used to check that speakup is working
properly, without the need for actually owning the hardware. Some
drivers require receiving characters, so we need a way to check that
this is working properly.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Link: https://lore.kernel.org/r/20201101183913.yg35cbqpn2ba6cew@function
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>