control the antenna control bits for the four TX series and the
TPC settings for TX series 1, 2, 3.
The specifics:
* The TPC setting for TX series 0 is handled in ctl0.
* TPC is currently disabled, so the per-packet TX power is
set via the global per-rate TX power register, not per packet.
* The antenna control bits don't matter for AR5416 and later
so they should stay 0 (which they currently do); they may
be set for Kite but as there's no TX diversity supported
at the moment (it requires the NIC to be built with an
external antenna switch, matching how antenna diversity
is done on legacy NICs), so again keep them 0.
This is in preparation for supporting per-rate TPC on the
AR5416 and later. The Kite (and soon to come Kiwi) code
sets ctl8-11 to 0x0, which doesn't have any effect at
the moment. When TPC is enabled it would result in the
second, third and fourth TX series attmpts to be done with
a TX power of 0. This commit doesn't change that; it'll
be followed up with some commits to properly set the TPC
registers appropriately.
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