net: calxedaxgmac: use raw i/o accessors in rx and tx paths

The standard readl/writel accessors involve a spinlock and cache sync
operation on ARM platforms with an outer cache. Only DMA triggering
accesses need this, so use the raw variants instead in the critical paths.

The relaxed variants would be more appropriate, but don't exist on all
arches.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: David S. Miller <davem@davemloft.net>