igbvf: after mailbox write, wait for reply

Two of the VF mailbox commands were not waiting for a reply from the PF,
which can result in a VF mailbox timeout in the VM for the next command.

Signed-off-by: Greg Edwards <gedwards@ddn.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>