gianfar: Fix eTSEC configuration procedure

Fix some bugs in the ethtool configuration functions:
* gfar_clean_rx_ring should not be called with interrupts disabled.
* Update last transmission time to avoid tx timeout.
* Delete redundant NETIF_F_IP_CSUM check in gfar_start_xmit
* Use netif_tx_lock_bh when reconfiguring the tx csum

Signed-off-by: Dai Haruki <dai.haruki@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>