rt2x00: Refactor rt2800 version constants.

The rt2800 version constants are inconsistent, and the version number don't
mean a lot of things anyway. Refactor the constants to have some more
meaningful names, and introduce and use some new helpers to check these
chipset revisions. At the same time rename to revision, as they are more
revision numbers rather than version numbers.

Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>