core: rename a simple function, fix re-arming triggers

zio_cset_is_self_timed() only returned true for input channels.  This
is needed, but the name is wrong: use zio_cset_early_arm() instead.

An output channel can be self-timed as well, so trigger_data_done
check the flag directly, not early_arm(). Every self-timed cset
must be rearmed when the trigger fired.

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
Acked-by: Federico Vaga <federico.vaga@gmail.com>