ravb: Group descriptor types used in Rx ring

The Rx ring can either be made up of normal or extended descriptors, not
a mix of the two at the same time. Make this explicit by grouping the
two variables in a rx_ring union.

The extension of the storage for more than one queue of normal
descriptors from a single to NUM_RX_QUEUE queues have no practical
effect. But aids in making the code readable as the code that uses it
already piggyback on other members of struct ravb_private that are
arrays of max length NUM_RX_QUEUE, e.g. rx_desc_dma. This will also make
further refactoring easier.

While at it, rename the normal descriptor Rx ring to make it clear it's
not strictly related to the GbEthernet E-MAC IP found in RZ/G2L, normal
descriptors could be used on R-Car SoCs too.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Paul Barker <paul.barker.ct@bp.renesas.com>
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>