zynq-gem: Use appropriate cache flush/invalidate for RX and TX

zynq-gem: Use appropriate cache flush/invalidate for RX and TX

The cache was only flushed before *transmitting* packets, but not
when receiving them, leading to an issue where new packets were
handed to the receive handler with old contents in cache. This
only happens when a lot of packets are received without sending
packages every now and then. Also flushing the receive buffers
in the transmit function makes no sense and can be removed.

Signed-off-by: Stefan Theil <stefan.theil@mixed-mode.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>