ath5k: use fixed antenna for tx descriptors

when using a fixed antenna we should use the antenna number in all tx
descriptors, otherwise the hardware will sometimes send the frame out on the
other antenna. it seems like the hardware does not always respect the default
antenna and diversity settings (esp.  AR5K_STA_ID1_DEFAULT_ANTENNA).

also i would like to note that antenna diversity does not always work correctly
on 5414 (at least) when only one antenna is connected: for example all frames
might be received on antenna A but still the HW tries to send on antenna B some
times, causing packet loss.

this is both verified with the antenna statistics output of the previous patch
and a spectrum analyzer.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>