rt2x00: Detect USB BULK in/out endpoints

Instead of hardcoding the used in/out endpoints
we should detect them by walking through all
available endpoints.

rt2800usb will gain the most out of this, because
the legacy drivers indicate that there are multiple
endpoints available.
However this code might benefit at least rt73usb as
well for the MIMO queues, and if we are really lucky
rt2500usb will benefit because for the TX and PRIO
queues.

Even if rt2500usb and rt73usb do not get better performance
after this patch, the endpoint detection still belongs to
rt2x00usb, and it shouldn't hurt to always try to detect
the available endpoints.

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