wl12xx: avoid starving the system hlid

Re-factor the Tx scheduler so that the system_hlid is taken into account
before restarting an iteration over the wlvifs. Previously this
hlid had a lower priority and would starve if some wlvif had many
packets.
In addition avoid iterating over wlvifs past last_wlvif when performing
the a second pass. If we had packets in those wlvifs they would have
been found earlier.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>