The NVMe specification recommends that orderly shutdown should just
write CC.SHN while the controller is still enabled rather than writing
CC.EN = 0 first.
This also allows removal of the now-unused nvme_ctrlr_disable() and
nvme_ctrlr_wait_for_ready() functions.
Change-Id: I4702ffda153f218ebb8ed92f0e36144b7ceded93
Signed-off-by: Daniel Verkamp <daniel.verkamp@intel.com>