rtl8xxxu: Pass tx_info to fill_txdesc in order to have access to retry count

In order to obtain retry count for a given rate we need to pass the
full struct ieee80211_tx_info to the function setting the rate in he
TX descriptor.

This uncovered a huge bug where the old code would use struct
ieee80211_rate.flags to test for rate parameters, which is always
zero, instead of the flags value from struct ieee80211_tx_rate.

Time to find a brown paper bag :(

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>