clockevents/drivers/em_sti: Migrate to new 'set-state' interface

Migrate em_sti driver to the new 'set-state' interface provided by
the clockevents core, the earlier 'set-mode' interface is marked
obsolete now.

This also enables us to implement callbacks for new states of clockevent
devices, for example: ONESHOT_STOPPED.

NOTE: This also drops a special check:

	if (old_mode == CLOCK_EVT_MODE_ONESHOT)
		em_sti_stop(p, USER_CLOCKEVENT);

as it doesn't look like that important. This driver only supports
ONESHOT and we can only move only to SHUTDOWN from ONESHOT and.
Also on second call (on shutdown), em_sti_stop() would return without
disabling the device again.

Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Magnus Damm <magnus.damm@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>