ui/gtk: don't try to redefine SI prefixes

Redefining SI prefixes is always wrong. 1s has per definition
1000ms. Remove the misnamed named constant and replace it with
a comment explaining the frequency to period conversion in two
simple steps. Now you can cancel out the unit mHz in the comment
with the implicit unit mHz in refresh_rate_millihz and see why
the implicit unit ms for update_interval remains.

Signed-off-by: Volker Rümelin <vr_qemu@t-online.de>
Message-Id: <20201213165724.13418-1-vr_qemu@t-online.de>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>