cassini: Removing dead CASSINI_MULTICAST_REG_WRITE

CASSINI_MULTICAST_REG_WRITE doesn't exist in Kconfig, therefore
removing all references for it from the source code.

Signed-off-by: Christoph Egger <siccegge@cs.fau.de>
Signed-off-by: David S. Miller <davem@davemloft.net>