modbus: reset wait semaphore before tx

A response returned after a request times out would increment the
semaphore and stay until the next request is made which will immediately
return when k_sem_take is called even before a response is returned. This
will once again have the same problem when the actual response arrives.
So the wait semaphore just needs to be reset before transmitting.

Signed-off-by: Abram Early <abram.early@gmail.com>
(cherry picked from commit 583f495)

subsys/modbus/modbus_core.c

@@ -137,6 +137,8 @@ void modbus_tx_adu(struct modbus_context *ctx)
 
 int modbus_tx_wait_rx_adu(struct modbus_context *ctx)
 {
+	k_sem_reset(&ctx->client_wait_sem);
+
 	modbus_tx_adu(ctx);
 
 	if (k_sem_take(&ctx->client_wait_sem, K_USEC(ctx->rxwait_to)) != 0) {