nfp: allocate a private workqueue for driver work

Since we grab pf->lock around pci_enable_sriov() we can no longer
safely queue work which may also grab that lock onto system workqueue.
pci_enable_sriov() will flush system workqueue as part to wait for VF
probing.

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