mac80211: move A-MSDU identifier to flags

This patch moves u8 amsdu_frame in ieee80211_txrx_data to the flags
section as IEEE80211_TXRXD_RX_AMSDU

Signed-off-by: Ron Rindjunsky <ron.rindjunsky@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>