[POWERPC] Only use H_BULK_REMOVE if the firmware supports it

The previous patch changing pSeries to use H_BULK_REMOVE broke the
JS20 blade, where the firmware doesn't support H_BULK_REMOVE.  This
adds a firmware check so that on machines that don't have H_BULK_REMOVE,
we just use the H_REMOVE call as before.

Signed-off-by: Paul Mackerras <paulus@samba.org>