ubusd: fix tx_queue linked list usage

ubusd: fix tx_queue linked list usage

Use the correct parameters for list_add_tail when enqueueing a message
in the tx_queue. Previously, list_add_tail(list, entry) was used instead
of list_add_tail(entry, list). Due to this, the list would only contain
the latest entry, and previously inserted entries were left dangling.

Reset the tx_queue offset after a message from the tx_queue has been sent
completely.

Signed-off-by: Alexander Van Parys <alexander.vanparys_ext@softathome.com>