[PATCH] bonding: cleanup comment for mode 1 IGMP xmit hack

Expand comment explaining MAC address selection for replicated IGMP
frames transmitted in bonding mode 1 (active-backup).  Also, a small
whitespace cleanup.

Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>