base: Fix bitstruct bug with assigning fields

Given a BitStruct 'foo', where bar was a BitStructField, assignment was
broken because it was using the C++ default behavior (copy the entire
word).

  foo.bar = other_foo.bar

This will now correctly use BitFieldInsert to update the 'bar' bitfield.

Change-Id: I1464462ffd4df22ad14a2e43a34f74d75ac2809a