i2c: guarantee that I2C_M_RD will be 0x0001 forever

There is code out there in user space and kernel space which relies on
I2C_M_RD being bit 0 to simplify their bit operations. Add a comment to
make sure this will never break. Do proper sorting of the defines while
we are here.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>