greybus: interface: Fetch and expose version of interface's firmware

The version of the currently running firmware on the module is useful
for userspace as it can be used to find if an update is available or
not. This patch fetches interface's version with a new control operation
and exposes the same in userspace.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>