rtl818x: make sure TX descriptor writes are done before kicking DMA

The TX descriptors are consumed by the HW using DMA.
Even if in the driver code the TX descriptor writes appears before
the HW "dma kick" register writes, the CPU may reorder them.

If this happens, the TX may not happen at all becase the "valid"
descriptor flag may have not been set yet.

This patch adds a write memory barrier to ensures the TX
descriptor is written before writing to the HW "dma kick" register.

Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>