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"), the obsolete context about desc_get_phys_addr was
introduced, so we need to recover it according to SDK commit abc6c0f2bd85
("net: axienet: added multichannel DMA support")

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