ipv6: Do not generate random IID only for RAWIP devices

This method is incorrect as it maybe incorrectly clearing some of
the bits needed for setting the IID.

CRs-Fixed: 2233026
Change-Id: I231e9b6a28a0d3f61f3bb19e311432a13bbd4db8
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>