nvme: don't intialize hwmon for discovery controllers

Discovery controllers usually don't support smart log page command.
So when we connect to the discovery controller we see this warning:
nvme nvme0: Failed to read smart log (error 24577)
nvme nvme0: new ctrl: NQN "nqn.2014-08.org.nvmexpress.discovery", addr 192.168.123.1:8009
nvme nvme0: Removing ctrl: NQN "nqn.2014-08.org.nvmexpress.discovery"

Introduce a new helper to understand if the controller is a discovery
controller and use this helper to skip nvme_init_hwmon (also use it in
other places that we check if the controller is a discovery controller).

Fixes: 400b6a7b

 ("nvme: Add hardware monitoring support")
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>