powerpc/pseries: address checkpatch warnings in dlpar_offline_cpu

Remove some stray blank lines, convert a printk to pr_warn, and
address a line length violation.

One functional change: use WARN_ON instead of BUG_ON in case H_PROD of
a ceded thread yields an unexpected result from the platform. We can
expect this code path to get uninterruptibly stuck in __cpu_die() if
this happens, but that's more desirable than crashing.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Fixes: b6db63d1 ("pseries/pseries: Add code to online/offline CPUs of a DLPAR node")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191016183611.10867-2-nathanl@linux.ibm.com