greybus: interface_block: move sysfs files into the interface_block.c file

No need to keep these out in sysfs.c, move them into the
interface_block.c file so that we can see them easier, and remove some
variable definitions by taking advantage of the attribute group macro.

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