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>