Fixup AGP_MODE{GET,SET}_RATE() macros. the agp 2.0 spec defines that the lowest

Fixup AGP_MODE{GET,SET}_RATE() macros. the agp 2.0 spec defines that the lowest
three bits are for the current agp rate. not the lowest two. Otherwise we
preclude agp 4x mode.

It's a bit different for agp 3.0, but we don't have the bits for that yet.

Tested by several as part of a larger diff.