net: rpl: Fix the statistics collection

The Kconfig.rpl file was missing option to actually collect
RPL statistics. Unified the config option name to be
CONFIG_NET_STATISTICS_RPL as there was two conflicting settings
in the code and both of them were missing from Kconfig file.

Change-Id: I4ce4fcbaa317b36cac315ea3b3f710fa7a344b25
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>