greybus: interface: we really are creating/destroying interfaces not modules.

rename gb_add_module     -> gb_add_interface
rename gb_remove_modules -> gb_remove_interfaces
rename gb_remove_module  -> gb_remove_interface

And move the function prototypes to interface.h, where they belong.

Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>