greybus: loopback: Fix broken synchonous test

loopback driver use the send_count variable to know the test progress.
The test may be stopped or change but this variable is never cleaned.
Such situation may break the next run.

Signed-off-by: Alexandre Bailon <abailon@baylibre.com>
Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>