isci: make a remote_node_context a proper member of a remote_device

A rnc object has the same lifetime as its associated remote_device.  It might
get re-initialized, but a remote device always has an rnc member.  Preparation
for unifying scic_sds_remote_device and isci_remote_device

Signed-off-by: Dan Williams <dan.j.williams@intel.com>