nfp: implement PCI driver shutdown callback

Device may be shutdown without the hardware being reinitialized, in
which case we want to ensure we cleanup properly.

This is especially important for kexec with traffic flowing.

The shutdown procedures resembles the remove procedures, so we can reuse
those common tasks.

Signed-off-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>