cmd: fpga: Clean wrong_parms handling

cmd: fpga: Clean wrong_parms handling

There is no reason to check parameters in separate switch. Check them
directly when they are read. Also there is no reason to check loadmk
case separately because fpga_data address must be non zero too.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>