nfp: fix variable dereferenced before check in nfp_app_ctrl_rx_raw()

'app' is dereferenced before used for the devlink trace point.
In case FW is buggy and sends a control message to a VF queue
we should make sure app is not NULL.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>