gianfar: Enable padding and Optimize the frame prepended bytes handling

The eTSEC can prepend up to 32 bytes to a received frame, usually for the
purpose of aligning the IP address to a word boundary, so this turns it on.

While we're in there, make the handling of the pre-frame bytes (padding and
Frame Control Block) cleaner.

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>