rtl8180: rationalize TX queues

rtl8180: rationalize TX queues

On currently supported HW there are four TX queues (three for normal
packets and one for beacons).

The driver uses just one TX queue, and declare to mac80211 to
support just one queue, but it allocates coherent memory for all
queues.
Furthermore the TX is code is written assumimg four queues exists,
and even if we decide to enable more queues in future, its mapping
rule to mac80211 is fixed.

This means we have memory waste on rtl8180/rtl8185, and we have also
not enough flexibility to add support for boards (rtl8187se) that
will use more queues.

This patch changes things in order to allocate coherent memory only
for the queues effectively used and it make it possible to specify
how to map hardware queues on mac80211 queues, that will be used
by rtl8187se code as soon it will be merged.

Note: even if the beacon queue is currently unused, this should
change, so I kept it.

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