speakup: Allow lower values for the flush parameter

speakup: Allow lower values for the flush parameter

Users needing it with the dectlk synth report that a 100ms flush delay
is still noticeable and prefer to set it to e.g. 10ms. This leaves the
default to 4000ms (since hitting it is a sign that the cable is faulty
and should be replaced), but allows to set it as short as 10ms.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Link: https://lore.kernel.org/r/20220205232957.bc6o6yyt5hitg754@begin
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>