devlink: require non-NULL ops for devlink instances

Commit 76726ccb ("devlink: add flash update command") and
commit 2d8dc5bb ("devlink: Add support for reload")
access devlink ops without NULL-checking. There is, however, no
driver which would pass in NULL ops, so let's just make that
a requirement. Remove the now unnecessary NULL-checking.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>