only turn off accept_ra_defrtr for rmnet interfaces

accept_ra_defrtr=0 on wifi interfaces breaks IPv6 for networks using RA.  For
wifi networks, the kernel sets up the default route as well as the interface
address (in other words: with defrtr=0, wifi interfaces get an address but no
default route).  This is not the case for the rmnet (ril) interfaces, as
Samsung's ril libraries configure the default IPv6 route after setting up the
PDP.