macb: Introduce a few barriers when dealing with DMA descriptors

macb: Introduce a few barriers when dealing with DMA descriptors

There were a few theoretical possibilities that the compiler might
optimize away DMA descriptor reads and/or writes and thus cause
synchronization problems with the hardware. Insert barriers where
we depend on reads/writes actually hitting memory.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>