greybus: bundle: rename interface.[c|h] to bundle.[c|h]

We are renameing the "interface" term to "bundle" so rename the files
before we start changing structure names to make it easier for people to
see what really is happening in the changes.

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