gianfar: Support NAPI for TX Frames

Poll the completed TX frames in gfar_poll().  This prevents the tx
completion interrupt from interfering with processing of received
frames.

We also disable hardware rx coalescing when NAPI is enabled.

Signed-off-by: Dai Haruki <dai.haruki@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>