JP2Grok: RasterIO(eBufType=GDT_Float32) fails

[rouault]

$ gdal_translate -if jp2grok autotest/gdrivers/data/jpeg2000/byte.jp2 out.tif -ot Float32
Input file size is 100, 100
0ERROR 6: DirectRasterIO: unsupported buffer data type Float32

CC @boxerab