intel: Silence "intel/intel_fbo.h:105:4: warning: comparison of unsigned expression...

intel: Silence "intel/intel_fbo.h:105:4: warning: comparison of unsigned expression < 0 is always false"

The test was of an enum, attIndex, which should be unsigned.  The
explicit check for < 0 was replaced with a cast to unsigned in an
assertion that attIndex is less than the size of the array it will be
used to index.

Reviewed-by: Eric Anholt <eric@anholt.net>