nfp: slice the netdev spawning function

We want to be able to create a special vNIC for control messages.
This vNIC should be created before any netdev is registered to allow
nfp_app logic to exchange messages with the FW app before any netdev
is visible to user space.  Unfortunately we can't enable IRQs until
we know how many vNICs we will need to spawn.

Divide the function which spawns netdevs for vNICs into three parts:
 - vNIC/memory allocation;
 - IRQ allocation;
 - netdev init and register.

This will help us insert the initialization of the control channel
after IRQ allocation but before netdev init and register.

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