s390/qeth: uninstall IRQ handler on device removal

When setting up, qeth installs its IRQ handler on the ccw devices. But
the IRQ handler is not cleared on removal - so even after qeth yields
control of the ccw devices, spurious interrupts would still be presented
to us.

Make (de-)installation of the IRQ handler part of the ccw channel
setup/removal helpers, and while at it also add the appropriate locking.
Shift around qeth_setup_channel() to avoid a forward declaration for
qeth_irq().

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>