nfp: set config bit (ifup/ifdown) on netdev open/close

When a netdev (PF netdev or representor) is opened or closed, set the
physical port config bit appropriately - which powers UP/DOWN the PHY
module for the physical interface.

The PHY is powered first in the HW/FW configuration step when opening
the netdev and again last in the HW/FW configuration step when closing
the netdev.

This is only applicable when there is a physical port associated with
the netdev and if the NSP support this. Otherwise we silently ignore
this step.

The 'nfp_eth_set_configured' can actually return positive values -
updated the function documentation appropriately.

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>