qeth: Remove unneeded structure member

The member irq_tasklet in the qeth_channel structure
is not referenced anymore and is removed from the
structure.

Signed-off-by: Thomas Richter <tmricht@linux.vnet.ibm.com>
Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>