cmd: efi_loader: Return CMD_RET_USAGE in case of not enough arguments

When typing 'bootefi' from U-Boot shell, nothing outputs. Like other
commands, return CMD_RET_USAGE so that it can print help message.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Alexander Graf <agraf@suse.de>