s390/vmur: remove unnecessary BUG statement

An existing BUG statement in vmur's interrupt handler triggers if:

  1. An online vmur device is removed (e.g. due to driver unload, manual
     unbind or channel-report words indicating hypervisor-side device
     removal)
  2. Device deactivation fails due to firmware/hypervisor error, leaving
     subchannel enabled for interrupts + drvdata=NULL
  3. Interrupt occurs

This situation is highly unlikely and not a clear indication of a
general system error that would warrant stopping the full Linux system.
Also it can be prevented completely by clearing the interrupt handler
when unsetting a vmur device's drvdata.

Replace the BUG statement in vmur's interrupt handler by clearing the
interrupt handler callback during device removal. Also move the initial
setting of the interrupt handler callback under lock for consistency
reasons.

Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>