mesh: Fix clean-up introduced bug

mesh: Fix clean-up introduced bug

Before the clean-up we were making a useless check of an otherwise
unused boolean (net->provisioner) to determine if we should decode with
device keys (which was incorrect). This was replaced by a check the
node type (node->provisioner). However, the check was incorrect in the
first place, and prevents provisioner nodes from decrypting non device
key messages.

Change-Id: Ia6c8a52afbfe03599884a29d71003a60231f71cb
Signed-off-by: anuj.bhumiya <anuj.bhumiya@samsung.com>