net: macb: Added dummy Tx priority Queue

Since the GEM controller prefers priority queue over regular
queue for Tx, there is a chance that the controller may treat
the default value in Tx priority queue pointer register is valid
and causing issues while interpreting the memory as descriptor.

So, created a dummy queue and marked the BD status as not ready.
Since, the hardware is not backwards compatible and this is the
correct change to maintain functionality.

Known issue: During the link auto negotiation in progress,
observed framing errors and these errors are not incrementing
after that for any data traffic like ping test and pktgen test.

Signed-off-by: Punnaiah Choudary Kalluri <punnaia@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>