net: xilinx: axiethernet: Check for queue full in transmit path

net: xilinx: axiethernet: Check for queue full in transmit path

As per DMA programming sequence, software must not resubmit a BD which
is already under the hardware control else results are undefined. In
iperf3 stress testing, we observed intermittent zero bandwidth and DMA
IP going into stall state. To fix this behavior, in the transmit path
add a check for queue space before adding new BD elements.

Signed-off-by: Radhey Shyam Pandey <radhey.shyam.pandey@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>