s2io.c: Shorten code line length by using intermediate pointers

Repeated variable use and line wrapping is hard to read.
Use temp variables instead of direct references.

struct fifo_info *fifo = &mac_control->fifos[i];
struct ring_info *ring = &mac_control->rings[i];
struct tx_fifo_config *tx_cfg = &config->tx_cfg[i];
struct rx_ring_config *rx_cfg = &config->rx_cfg[i];

Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Sreenivasa Honnur <sreenivasa.honnur@neterion.com>
Signed-off-by: David S. Miller <davem@davemloft.net>