ixgbe: Move max frame size and Rx buffer length configuration into a function

This change consolidates all of the Rx max frame size and Rx buffer length
configuration into a single function.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>