cmd: fpga: Move parameter checking for loadfs/loads

cmd: fpga: Move parameter checking for loadfs/loads

There is no reason to check parameters in separate switch before main
one. This patch is simplifying error path and checking parameters right
after assignment.

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