NTB: reset tx_index on link toggle

If the NTB link toggles, the driver could stop receiving due to the
tx_index not being set to 0 on the transmitting size on a link-up event.
This is due to the driver expecting the incoming data to start at the
beginning of the receive buffer and not at a random place.

Signed-off-by: Jon Mason <jon.mason@intel.com>