Issue with transmission synchronisation functions

[Deniz-Eren]

There seems to be a deadlock issue with the mutex and/or cond_var setup in netif_stop_queue(). The issue could be in combination to how the Linux drivers callback the synchronisation functions also. The issue presents itself when very high transmission rate is applied under non-realistic test conditions. It is suspected the synchronisation functions of the transmission function netif_tx() is problematic. The symptom is the loss of receive ability when this deadlock happens.

Refactor netif_tx() and synchronisation interface functions netif_start_queue(), netif_queue_stopped(), netif_wake_queue() and netif_stop_queue(). The issue can be replicated thus the fix can be verified.