r6040: fix ifconfig down and freeing of tx/rx descriptors

This patch fixes warnings and such traces that appear when doing
an ifconfig down on the interface:

WARNING: at arch/x86/kernel/pci-dma.c:376 dma_free_coherent+0x40/0x7d()
Modules linked in:

Signed-off-by: Joe Chou <joe.chou@rdc.com.tw>
Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>