nfp: process control messages in workqueue in flower app

Processing of control messages is not time-critical and future processing
of some messages will require taking the RTNL which is not possible
in a BH handler. It seems simplest to move all control message processing
to a workqueue.

Signed-off-by: Simon Horman <simon.horman@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>