mac80211: fix wiphy supported_band access

Fix wiphy supported_band access in tx radiotap parsing introduced
in commit 5ec3aed9ba4c ("mac80211: Parse legacy and HT rate in
injected frames"). In particular, info->band is always set to 0
(IEEE80211_BAND_2GHZ) since it has not assigned yet.
This cause a kernel crash on 5GHz only devices.
Move ieee80211_parse_tx_radiotap() after info->band assignment

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi83@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>