net: ethernet: mtk_wed: rely on __dev_alloc_page in mtk_wed_tx_buffer_alloc

Simplify the code and use __dev_alloc_page() instead of __dev_alloc_pages()
with order 0 in mtk_wed_tx_buffer_alloc routine

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>