cmd: fpga: Reorder the arguments parsing code

This patch modifies the arguments parsing code by parsing
based on requested operation for fpga loadfs and then
parses the most common/basic args for other fpga load
commands. This makes it easy for new command extensions
or additions especially the commands with more args.

Signed-off-by: Siva Durga Prasad Paladugu <siva.durga.paladugu@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>