gianfar: Poll only active Rx queues

Split the napi budget fairly among the active queues only, instead
of dividing it by the total number of Rx queues assigned to the
given interrupt group.
Use the h/w indication field RXFi in rstat (receive status register)
to identify the active rx queues from the current interrupt group
(i.e. receive event occured on ring i, if ring i is part of the current
interrupt group). This indication field in rstat, RXFi i=0..7,
allows us to find out on which queues of the same interrupt group
do we have incomming traffic once we entered the polling routine for
the given interrupt group. After servicing the ring i, the corresponding
bit RXFi will be written with 1 to clear the active queue indication for
that ring.

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