MACB: clear transmit buffers properly on transmit underrun

Initially transmit buffer pointers were only reset. But buffer
descriptors were possibly still set as ready, and buffer in upper
layer was not freed. This caused driver hang under big load.  Now
reset clean properly the buffer descriptor and freed upper layer.

Signed-off-by: Gregory CLEMENT <gclement00@gmail.com>
Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>