ixgbe: declare ixgbe_mac_operations structures as const

Declare ixgbe_mac_operations structures as const as they are only stored
in the mac_ops field of ixgbe_info structure. This field is of type
const and therefore ixgbe_mac_operations structure can be made const
too.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>