ieee1394: ohci1394: don't schedule IT tasklets on IR events

Bug noted by Pieter Palmers:  Isochronous transmit tasklets were
scheduled on isochronous receive events, in addition to the proper
isochronous receive tasklets.

http://marc.info/?l=linux1394-devel&m=119783196222802

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>