fix some dodgy FieldUnit conversions

the FieldUnit enum was being converted in some dodgy ways and
in some places the MapUnit enum values were being used.

Change-Id: Ic9aacb84058d1c14c3a4a79ef6676082df9a7270
cleanup usage of MapUnit enum

some places were using the wrong enum constant, and some places were
unnnecessarily converting to sal_Int16

Change-Id: I754993533c840cd33106820d99af2dc951f26a80