ixgbe: Clean up unneeded set in ixgbe_if_multi_set

We don't need to set the bits here since the if/else if/else statements
fully cover setting these bit pairs.

Reported by: markj
Reviewed by: markj, erj
Approved by: Intel Networking
MFC aftter: 1 week
Differential Revision: https://reviews.freebsd.org/D29827