net/cadence_gem: Fix register w1c logic

net/cadence_gem: Fix register w1c logic

This write-1-clear logic was incorrect. It was always clearing w1c
bits regardless of whether the written value was 1 or not. i.e. it
was implementing a write-anything-to-clear strategy.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: ed905b04d3343966ded425f06aa2224bc7a35b59.1386136219.git.peter.crosthwaite@xilinx.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>