HFA driver: re-writing statistics with 8-bit base data value

[nflood]

When re-writing statistics in an existing HFA file, i.e. it already has some statistics, but they needed to be re-calculated (e.g. after raster editing in Imagine), the only case dealt with in hfafield.cpp was when the basedata value was a float64. However, Imagine can also write this as an 8-bit value, and when GDAL tries to re-write it, it doesn't have a case for that, and results in an error message, and the value is not re-written. The attached patch adds a case for the basedata field being 8-bit, and handles it otherwise the same way.

The patch is against the current 1.10.0 release. It was originally done against 1.9.0, and has been in use for over a year without further problems.