rtl8180: be paranoid in stopping unused queues.

HW should never attempt to perform DMA for unused queues.
For rtl8187se this is ensured by setting a dedicated register at
init time, before enabling TX.

In rtl8180/5 the register is only written at the first TX (because
in rtl8180/5 it serves also to kick DMA for used queues).
This should be enough, but it's worth to add a register write at
init time, before enabling TX.

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