ixgbe: Use rx buffer length from rx ring for configuring rscctl

While configuring rscctl use rx buffer length from rx ring structure
instead of passing rx_buf_len to ixgbe_configure_rscctl

Signed-off-by: Mallikarjuna R Chilakala <mallikarjuna.chilakala@intel.com>
Acked-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>