i2o: destroy event queue only when drv->event is set

i2o_driver_register() initalizes event queue for driver only when
drv->event is set.  So similarly the event queue should be destroyed only
when drv->event is set in the error path.  Otherwise destroy_workqueue()
will called with NULL.

Cc: Markus Lidel <Markus.Lidel@shadowconnect.com>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>