powerpc/powernv: Use CPU-endian hub diag-data type in pnv_eeh_get_and_dump_hub_diag()

The hub diag-data type is filled with big-endian data by OPAL call
opal_pci_get_hub_diag_data(). We need convert it to CPU-endian value
before using it. The issue is reported by sparse as pointed by Michael
Ellerman:

  eeh-powernv.c:1309:21: warning: restricted __be16 degrades to integer

This converts hub diag-data type to CPU-endian before using it in
pnv_eeh_get_and_dump_hub_diag().

Fixes: 2a485ad7c88d ("powerpc/powernv: Drop PHB operation next_error()")
Cc: stable@vger.kernel.org # v4.1+
Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Reviewed-by: Russell Currey <ruscur@russell.cc>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>