nfp: remove RX queue pointers

NFP6000 doesn't use queue pointers/doorbells for RX, it uses
'done' bit in descriptors.  Remove the pointers from data structures.
Since we are saving space in rx_ring structure make fields we
previously compressed to 16bits word size again.

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