rt2x00: Clear queue entry flags during initialization

When the queues are being initialized the entry flags fields must be
reset to 0. When this does not happen some entries might still be
marked as "occupied" after an ifdown & ifup cycle which would trigger
errors when the entry is being accessed:

	phy0 -> rt2x00queue_write_tx_frame: Error - Arrived at non-free entry in the non-full queue 0.
	Please file bug report to http://rt2x00.serialmonkey.com.

This also fixes the mac80211 warning:

	------------[ cut here ]------------
	WARNING: at net/mac80211/tx.c:1238 ieee80211_master_start_xmit+0x30a/0x350 [mac80211]()

which was triggered by the queue error.

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