gianfar: Refactor config coalescing calls for all queues

The only place where gfar_configure_coalescing is called
with an actual bitmask (other than 0xff) is in gfar_poll
(on the hot path). So make gfar_configure_coalescing()
static for the buffer processing path, and export
gfar_configure_coalescing_all() for the remaining cases
that require to set coalescing for all the queues at once
(on the slow path).

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