pimd: There is nothing to do with a WRVIFWHOLE for iifp of pimreg

pimd: There is nothing to do with a WRVIFWHOLE for iifp of pimreg

When the WRVIFWHOLE callback is made with a iifp of the pimreg
device we *know* that the packet is a PIM Register packet
( see net/ipv4/ipmr.c for kernel behavior ).  As such
we know that we will shortly read the pim register packet
and handle it through those mechanics.  There is nothing
to do here so we can move along.

Ticket: CM-27729
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>