When multicasting the driver sets the number of group addresses using
the count from the previous set multicast command. In general this means
you have to set the multicast addresses twice to get the behaviour you
want.

If we were multicasting, and reduce the number of addresses we are
multicasting to, then the driver would write uninitialised data from the
stack into the group addresses to multicast to.

Only write the multicast addresses we have specifically set.
Signed-off-by: David Kilroy <kilroyd@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>