igc: correctly update RCTL when changing multicast filters.

Fix clearing of bits in RCTL for the non-bpf/non-allmulti case.

Update RCTL after modifying the multicast filter registers as per
the Linux driver.

This fixes LACP on igc interfaces, where incoming LACP multicast control
packets were being dropped.

Obtained from: Rubicon Communications, LLC ("Netgate")

Set up an LACP session with igc interfaces and verify all links are seen.