drivers: eSPI: Incorrect handling of OOB registers leads to report wrong OOB packet len

[albertofloyd]

During eSPI sample OOB transactions are executed to retrieve temperature from eSPI host.
This occurs successfully in eSPI bus level and buffer with data is returned.
However, bytes received is always reported as 0, despite providing the right response data

To Reproduce
Steps to reproduce the behavior:

1. cd samples/drivers/espi

2. west build -c -b mec15xxevb_assy6853

3. west flash

4. see error on ouput

        - "espi: OOB transaction completed rcvd: **0 bytes**"
        - "espi: Postcode fe"
        - "espi: Postcode 7f"
        - "espi: eSPI sample completed"
   

Expected behavior
Apart from correct eSPI traffic, correct value should be there

Impact
Incorrect handling of eSPI host temperature for thermal operations