iucv: get rid of in_atomic() use.

This BUG_ON is not needed, since all (debug) checks are also done
in smp_call_function() which gets called by this function.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Ursula Braun <braunu@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>