net: devlink: skip info_get op call if it is not defined in dumpit

In dumpit, unlike doit, the check for info_get op being defined
is missing. Add it and avoid null pointer dereference in case driver
does not define this op.

Fixes: f9cf2288 ("devlink: add device information API")
Reported-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>