rt2x00: Implement watchdog monitoring

Implement watchdog monitoring for USB devices (PCI support can
be added later). This will determine if URBs being uploaded to
the hardware are actually returning. Both rt2500usb and rt2800usb
have shown that URBs being uploaded can remain hanging without
being released by the hardware.
By using this watchdog, a queue can be reset when this occurs.
For rt2800usb it has been tested that the connection is preserved
even though this interruption.

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