nfp: break up nfp_net_{alloc|free}_rings

nfp_net_{alloc|free}_rings contained strange mix of allocations
and vector initialization.  Remove it, declare vector init as
a separate function and handle allocations explicitly.

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