net: axienet: fix compile errors

When merging mainline commit bb193e3d ("net: axienet: fix for TX busy
handling"), the old function axienet_check_tx_bd_space was brought back,
so we need to delete it to fix compile error.

When merging mainline commit 95978df6

 ("net: axienet: add missing
memory barriers"), some part about dma_rmb is lost, add it back.

Signed-off-by: Quanyang Wang <quanyang.wang@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>