Fix GEMV kernels not clamping the output

* The flag is set incorrectly that disables activation function
  in the GEMV asssembly kernel.
* Test is updated to drive the clamping parameters properly.
  - The clamping parameter is set to reduce 20% the dynamic range
    of the output.

Resolves: KLEIDIAI-545

Signed-off-by: Viet-Hoa Do <viet-hoa.do@arm.com>

Approved-by: Jakub Sujak <jakub.sujak@arm.com>