Stop writing a bogus IEEE80211_RADIOTAP_DBM_TX_POWER field.

Stop writing a bogus IEEE80211_RADIOTAP_DBM_TX_POWER field.

We were setting this field to 0 as we had no easy way of determining
what TX power was used to transmit a frame. However, by including this
field and setting it to 0dBm we are saying that each frame is being
transmitted at 1mW, which is incorrect. Better to simply not write the
field at all, however this introduces the need for padding in the TX
header to ensure that the TX_FLAGS field is aligned to a 16-bit
boundary.

git-svn-id: http://madwifi-project.org/svn/madwifi/trunk@3014 0192ed92-7a03-0410-a25b-9323aeb14dbd