qla3xxx: fix: Remove unused set_multicast function.

This device is one side of a two-function adapter (NIC and iSCSI).
Promiscuous mode setting/clearing is not allowed from the NIC side.

Signed-off-by: Ron Mercer <ron.mercer@qlogic.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>