pseries: ibmebus.c: convert to use BUS_ATTR_WO

We are trying to get rid of BUS_ATTR() and the usage of that in
ibmebus.c can be trivially converted to use BUS_ATTR_WO(), so use that
instead.

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Tyrel Datwyler <tyreld@linux.vnet.ibm.com>
Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>