block, bfq: fix comments on __bfq_deactivate_entity

Comments on function __bfq_deactivate_entity contains two imprecise or
wrong statements:
1) The function performs the deactivation of the entity.
2) The function must be invoked only if the entity is on a service tree.

This commits replaces both statements with the correct ones:
1) The functions updates sched_data and service trees for the entity,
so as to represent entity as inactive (which is only part of the steps
needed for the deactivation of the entity).
2) The function must be invoked on every entity being deactivated.

Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>