misc: zynq_slcr: Fix MMIO writes

The /4 for offset calculation in MMIO writes was happening twice giving
wrong write offsets. Fix.

While touching the code, change the if-else to be a short returning if
and convert the debug message to a GUEST_ERROR, which is more accurate
for this condition.

Cc: qemu-stable@nongnu.org
Cc: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>