cmd: host: Mandate the filename parameter in the 'bind' command

At present the host bind command does not require filename to be
provided. When it is not given NULL is passed to the host device
driver, which ends up failure afterwards.

Change to mandate the filename so that it is useful.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Simon Glass <sjg@chromium.org>