fix InterfaceController::setIPv6AddrGenMode(INetd::IPV6_ADDR_GEN_MODE_EUI64)

to actually do something... writing an empty string is a no-op by definition,
furthermore the way to switch to EUI64 mode is via 'addr_gen_mode' file,
not via the stable_secret file.

Test: builds and TreeHugger
Bug: 138428295
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I77515cfec315136e0d8641fa1457917cc1efe70d