s390: Remove BKL from prng

cycle_kernel_lock() was added during the big BKL pushdown. It should
ensure the serializiation against driver init code. In this case there
is nothing to serialize. Remove it.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
LKML-Reference: <20091010153349.601625576@linutronix.de>
Acked-by: Jan Glauber <jang@linux.vnet.ibm.com>