w1: ds1wm: make endian clean and use standard io memory accessors
o Make endian clean, make HW-endianness configurable.
o Use ioread*, iowrite* instead of __raw_readb,__raw_writeb
to also use memory-barriers when accessing HW-registers.
We do not want reordering to happen here.
Both changes are tightly coupled, so I do them in one patch