brcmsmac: Use IEEE 802.11 AC levels for pktq precedence levels

The mac80211 tx queues and brcmsmac DMA fifos both map directly to AC
levels. Therefore it's much more straightforward to queue tx frames and
choose the tx fifo based on the mac80211 queue instead of mapping 802.1D
priority tags to precedence levels then back to AC levels. mac80211
already maps the 802.1D levels to the appropriate AC levels and queues
management frames at the maximum priority, so the results should be
identical.

One functional change resulting from this patch is that AMPDU retries no
longer get a priority boost to queue them ahead of packets with the same
priority already in the tx queue. This behavior will be restored (in
effect at least) in a later patch when the tx queue is removed.

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Arend van Spriel <arend@broadcom.com>
Tested-by: Daniel Wagner <wagi@monom.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>