gianfar: Remove 'maybe-uninitialized' compile warning

Warning message:
warning: 'budget_per_q' may be used uninitialized in this function

budget_per_q won't be used uninitialized since the only time
it doesn't get initialized is when entering gfar_poll with
num_act_queues == 0, meaning rstat_rxf == 0, in which case
budget_per_q is not utilized (as it has no meaning).
Inititalize budget_per_q to 0 though to suppress this compile
warning.

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