net: core: Do not handle RPL options if RPL not enabled

If RPL is not enabled do not handle RPL options in IPv6 extra header.
Issues will arise if we skip reading those bytes and continue further.

E.g. Upper layer protocol like ICMPv6 echo request does not prepare
proper echo reply if we skip like this. It doesn't know what options
need to be updated in reply header.

Change-Id: I250672a54d4c7f60727313f6e7f8fe902685d0c5
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>