nfp: add async reconfiguration mechanism

Some callers of nfp_net_reconfig() are in atomic context so
we used to busy wait for commands to complete.  In worst case
scenario that means locking up a core for up to 5 seconds
when a command times out.  Lets add a timer-based mechanism
of asynchronously checking whether reconfiguration completed
successfully for atomic callers to use.  Non-atomic callers
can now just sleep.

The approach taken is quite simple because (1) synchronous
reconfigurations always happen under RTNL (or before device
is registered); (2) we can coalesce pending reconfigs.
There is no need for request queues, timer which eventually
takes a look at reconfiguration result to report errors is
good enough.

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