net: ravb: Count packets instead of descriptors in R-Car RX path

The units of "work done" in the RX path should be packets instead of
descriptors.

Descriptors which are used by the hardware to record error conditions or
are empty in the case of a DMA mapping error should not count towards
our RX work budget.

Also make the limit variable unsigned as it can never be negative.

Fixes: c156633f ("Renesas Ethernet AVB driver proper")
Signed-off-by: Paul Barker <paul.barker.ct@bp.renesas.com>
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>