rt2x00: Use IEEE80211_TX_CTL_MORE_FRAMES flag

Check the IEEE80211_TX_CTL_MORE_FRAMES flag to help determining
if the DMA queue should be kicked. At the moment this is combined
with the ieee80211_has_morefrags() but we might remove that later.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>