ixgbevf: Fix bungled declaration of ixgbevf_mbx_ops

In two places storage for mbx_ops is misidentified as type
ixgbe_mac_operations.

Reported-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Greg Rose <gregory.v.rose@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>