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 multicasti
control packets were being dropped.

Approved by: re (gjb)
Obtained from: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit 5a3eb6207a353c3a18da8abcf00a2d75276dd29e)
(cherry picked from commit 73e1138208a5888aaabfc44d8e48aa04554c9146)