net/mlx5: remove duplicated reference of Tx doorbell

net/mlx5: remove duplicated reference of Tx doorbell

The Tx doorbell has different virtual addresses per process.
The secondary process takes the UAR physical page ID of the primary and
mmap it to its own virtual address.
The primary doorbell references were saved in two shared memory
locations: the TxQ structure and a dedicated doorbell array.

Remove the doorbell reference from the TxQ structure and move the
primary processes to take the UAR information from the primary doorbell
array.

Cc: stable@dpdk.org
Signed-off-by: Michael Baum <michaelba@nvidia.com>
Reviewed-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>
Acked-by: Matan Azrad <matan@nvidia.com>