Clean-up: consumerd: use a specific status code for get_next_subbuffer

Clean-up: consumerd: use a specific status code for get_next_subbuffer

The implementation of "get next subbuffer" of the user space and kernel
tracers return different error codes (-ENODATA and -EAGAIN) which are
are confusing to handle in the generic code.

Since the difference between -ENODATA and -EAGAIN makes no material
difference in the current consumerd implementation, those conditions
are abstracted by a common GET_NEXT_SUBBEFFER_STATUS_NO_DATA.

Otherwise, the callers handle 'OK' and the generic 'ERROR' condition
which makes the transport of more specific "errno" values useless for
the moment.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ibdb2837396e4b8cd291ffd80f6ca59b39ce3f707