gianfar: Add backwards compatible Single Queue mode polling

Older Single Queue (SQ_SG_MODE) devices like TSEC (i.e. mpc83xx)
don't feature the frame receive indication bits (RXF) in RSTAT.
For these and for the rest of the SQ_SG_MODE devices, provide the
appropiate polling routine that handles a single pair of Rx/Tx
BD rings, removing the overhead incurred by the multiple queues/
multiple interrupt group devices (veTSEC/ eTSEC2.0 devices).
So this is primarily a fix for the TSEC devices.

Signed-off-by: Claudiu Manoil <claudiu.manoil@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>