wireless/rt2x00: don't use PREPARE_WORK in rt2800usb.c

PREPARE_[DELAYED_]WORK() are being phased out. They have few users
and a nasty surprise in terms of reentrancy guarantee as workqueue
considers work items to be different if they don't have the same work
function.

Update rt2800usb.c to use INIT_WORK() instead of PREPARE_WORK(). As
the work item isn't in active use during rt2800usb_probe_hw(), this
doesn't cause any behavior difference.

It would probably be best to route this with other related updates
through the workqueue tree.

Only compile tested.

Signed-off-by: Tejun Heo <[e-mail]>
Cc: Ivo van Doorn <[e-mail]>
Cc: Gertjan van Wingerde <[e-mail]>
Cc: Helmut Schaa <[e-mail]>
Cc: [e-mail]

drivers/net/wireless/rt2x00/rt2800usb.c

···
 	/*
 	 * Overwrite TX done handler
 	 */
-	PREPARE_WORK(&rt2x00dev->txdone_work, rt2800usb_work_txdone);
+	INIT_WORK(&rt2x00dev->txdone_work, rt2800usb_work_txdone);
 
 	return 0;
 }