nvme: only allow entering LIVE from CONNECTING state

The fabric transports and also the PCI transport are not entering the
LIVE state from NEW or RESETTING. This makes the state machine more
restrictive and allows to catch not supported state transitions, e.g.
directly switching from RESETTING to LIVE.

Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Daniel Wagner <wagi@kernel.org>
Signed-off-by: Keith Busch <kbusch@kernel.org>