firmware: arm_sdei: clean up sdei_event_create()

Function sdei_event_find() is always called in sdei_event_create(), but
it is already called in sdei_event_register(). This code is trying to
avoid a double-create of the same event, which can't happen as we still
hold the sdei_events_lock. We can remove this needless sdei_event_find()
call.

Signed-off-by: Liguang Zhang <zhangliguang@linux.alibaba.com>
[expanded commit message]
Signed-off-by: James Morse <james.morse@arm.com>