greybus: power_supply: fix update interval check at request handler

We use the update interval to control the remove path and we set it to
zero when we do not want to have more updates in transit. That means
that the check in the request handler needs to be for interval update
zero to discard the newly received request and not the other way around
like it is.

This will fix the issue that all incoming requests were being discard.

Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>