ar9170: handle otus' A-MPDU density definitions

Otus uses slightly different set of "Minimum MPDU Start Spacing" values
than the 802.11n D2.0 specifies. (the whole table is shifted by one and
therefore the 16us spacing is not officially available!)

And while we're at it, we also initialize our MAC's density register.
So, this annoying _feature_ will not break TX A-MPDU later.

Signed-off-by: Christian Lamparter <chunkeey@web.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>