net: mvneta: avoid possible cache misses in mvneta_rx_swbm

Do not use rx_desc pointers if possible since rx descriptors are stored in
uncached memory and dereferencing rx_desc pointers generate extra loads.
This patch improves XDP_DROP performance of ~ 110Kpps (700Kpps vs 590Kpps)
on Marvell Espressobin

Analyzed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>