MAC DBDMA: store register values in native endianness

Store the register values in native endianness, by dropping all the
endianness conversion functions, and converting the endianness in
dbdma_readl/dbdma_writel instead.

Also guard the endianness conversion with TARGET_WORDS_BIGENDIAN to
simulate the backward connection of the bus.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
