xilinx_emacps: Fix handling of skb fragments (3/3)

xilinx_emacps: Fix handling of skb fragments (3/3)

When a Tx packet is fragmented, the controller sets USED=1 only in the
first buffer. This means that xemacps_tx_poll() cannot "preserve" the
USED bit (which may be 0), but must set USED=1 explicitly once the
packet has been transmitted.

Signed-off-by: Thomas Betker <thomas.betker@rohde-schwarz.com>
Tested-by: Anirudha Sarangi <anirudh@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>