mac80211: fix rate control initialisation

In commit 2e92e6f2 ("mac80211: use rate
index in TX control") I forgot to initialise a few new variables to -1 which
means that the rate control algorithm is never triggered and 0 is used as
the only rate index, effectively fixing the transmit bitrate at the lowest
supported.

This patch adds the missing initialisation.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Bisected-by: Sitsofe Wheeler <sitsofe@yahoo.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>