fix quantization zero-point generation

* for int8 and uint8 only
* not for int16
* pass all data types to the quantizaion generator functions as some
  operators (all convolutions, fully_connected) may use different types
  for the input and weight tensors
* also allows input type filtering for convolution operators

Change-Id: Iea3d00f03807a8db35b40d4e8988929ec6549b44
Signed-off-by: Les Bell <les.bell@arm.com>