rt2x00: descriptors

Remove txd and rxd descriptor structures. All access to the descriptors should be done by treating the descriptor as a register. For this we do add a desc structure containing a array, and several methods for reading and writing to that array.
This is prepares rt2x00 for the overhaul of the byteordering mechanism.

Signed-off-by Ivo van Doorn <[email protected]>

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http://www.mendiosus.nl/rt2x00/03_descriptor.diff