gelic: TX descriptor handling cleanup

        - Emitted return value of NETDEV_TX_LOCKED when DMA map or kick
          failure.
          Now it would free the skb, update drop packet statistics
          and return OK. Requested from Jeff Garzik.
        - Enable tx queue if number of free descriptors are more than 2
        - Fixed descriptor leak if dma map for second descriptor failed
        - Stopped calling xmit handler from interrupt handler in order
          to recheck tx queue.  Instead, call appropriate helper functions.
Signed-off-by: Masakazu Mokuno <mokuno@sm.sony.co.jp>
Signed-off-by: Jeff Garzik <jeff@garzik.org>