gianfar: Optimize interrupt coalescing configuration

Store the interrupt coalescing values in the form in which they will be
written to the interrupt coalescing registers.  This puts a little overhead
into the ethtool configuration, and takes it out of the interrupt handler

Signed-off-by: Dai Haruki <dai.haruki@freescale.com>
Acked-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>