fs: fix do_fsload() handling of optional arguments

Most arguments to the shell command do_fsload() implements are optional.
Fix the minimum argc check to respect that. Cater for the situation
where argv[2] is not provided.

Enhance both do_fsload() and do_ls() to check the maximum number of
arguments too. While this check would typically be implemented via
U_BOOT_CMD()'s max_args parameter, if these functions are called
directly, then that check won't exist.

Finally, alter do_ls() to check (argc >= 4) rather than (argc == 4) so
that if the function is enhanced to allow extra arguments in the future,
this test won't need to be changed at that time.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>